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You can get 1.3 of them for less than $50. Or 3.3 for $90. We're talking about megapixels here. As digital cameras have come down in price, and as their resolution increases, more and more of us are becoming digital photographers. Each picture that we take includes millions of pixels -- short for picture elements -- tiny dots of color that when combined create a photographic image. The more pixels, the more detailed the image. And the more pixels, the larger the file. As Hamlet remarked, "Ay, there's the rub," for in that mass of megapixels what files we save must give us pause. Here are some true-life examples that happened this week:

"I sent her the pictures by email," said Will in the next room, "but she didn't get them." He had attached 56 images from his digital camera to an email message to a colleague. They did not go through because his email server refused to send them along. Why? Because the attached files amounted to more than 100 megabytes of information.Many email servers limit the size of attachments to one or two megabytes. Will's pictures, traveling through a 56k modem, would have taken five hours to download onto his colleagues' computer.

"This slide show is taking for ever to load," complained Sally as she prepared to deliver her oral report to the class. The hourglass spun and the disk drive whirred. It took two computer crashes and much consternation before she could proceed with her presentation on the cleanup of the Nashua River, illustrated with photos taken with the school's new 6-megapixel digital camera. "Yes, I took the pictures right from the camera and inserted them into PowerPoint. They came in really huge -- I had to grab the corners and shrink each one to fit." Sally's PowerPoint file, containing 20 images, was almost 35 megabytes in size, and taxed the capabilities of the classroom computer.

"How come I can only see the sky," asked the student in the web design course. "All that shows on the web page is some blue sky a a wisp of cloud. The rest of the picture is missing. And it takes forever for the page to load. What's wrong?" The student had taken a photo of the school with her digital camera, and inserted the image onto a web page. The picture contained so many pixels that it extended well beyond the borders of their browser window, and all that was visible on the screen was a corner of the sky.

All three of these computer-users were misled by megapixels. They did not realize that the pictures from their digital cameras were not usable for the work they needed to do. While millions of pixels are important when you want to print pictures on paper, they may get in the way of distributing pictures online. How can you avoid megapixel madness when it comes to sharing pictures across the web?

Learn about resolution

The display that you are looking at right now is made up of many little tiny points of light. They are called pixels, just as with the digital image from your camera. Your computer can display 72 to 84 pixels per inch, enough to make a photograph seem continuous. But if you look at your display through a magnifying glass, you'll see the individual dots. Now take a look at a paper print from a digital camera. Look as closely as you can -- you'll need a microscope to see the pixels. That's because the photo printer can create 600 or more pixels per inch. The printer has a much higher resolution than the computer display.

To maintain good quality for printing on paper, the image file needs to contain as many pixels as possible, up to the number that the printer can handle. My 2-megapixel Canon SureShot digital camera takes pictures at a resolution of 180 pixels per inch, with each photo being 1600 pixels wide and 1200 pixels high. This image contains 1,920,000 pixels, and before compression consumes 5.5 megabytes of file space. You can see how big these files can be! The photos from a 4- or 6-megapixel camera contain even higher resolution and more data.

I need all these pixels to get high-quality images when I print them to paper. But when I want to show them on the computer screen, with its lower resolution, those megapixels can slow me down. In the examples above, the plethora of pixels indeed got in the way, and made the work difficult if not impossible. For images on the web, or in PowerPoint, or for viewing on a computer screen, anything beyond 72 pixels per inch is wasted data, that disable efficient transmission.

To make the best use of the pictures that come from your digital camera, you must learn to manage their resolution. One way to do this is to snap your photos in the camera with less resolution. Most cameras let you reduce the resolution, or quality of the pictures you take. These pictures take up less room, work better in online applications, and allow you to take more pictures with a single memory chip. But while these images will work well online, they will not print with the quality you expect.

If you want to take pictures that show good quality for printing, and yet use these pictures online as well, you'll have to learn to adjust their resolution. What you will do is keep two copies of each image: one high-resolution copy for printing on paper; and another, low-resolution copy for use online. Here are some ways to do this:

Both Macintosh OS X and Windows XP come with built-in image management software that lets you import the pictures from your camera, save them to disk in full quality, and then later to export a low-resolution copy suitable for use with PowerPoint, web pages, or emailing.

With Windows XP, the application is called PictureIt. In most cases, PictureIt launches automatically when you attach your digital camera. When you choose to import the pictures from the camera, they are saved in the My Pictures folder inside the My Documents folder, in the PictureIt Library. They will be saved just as they came from the camera, in most cases in a high-resolution format. To change the resolution to a value more appropriate to online use, double-click the image to open it, then follow these steps:

1. On the Format menu, click Resize Image.
2. Click Resolution
3. Enter 72 pixels per inch for the resolution.
4. Click Done.
5. On the File menu, click Save As. The Save As dialog box opens.
6. Click the folder where you want to save this lower-resolution copy of the original.
7. In the File name box, enter a name for the copy.
8. Click Save.

The user's guide for PictureIt is available online.

With Macintosh OS X, the application is called iPhoto. In most cases, iPhoto launches itself automatically when you attach your digital camera. When you choose to import the pictures from the camera, they are saved in the iPhoto Library in the Pictures folder in the Finder. They will be saved just as they came from the camera, in most cases in a high-resolution format. Once imported, you can see their thumbnails in the iPhoto display window. To export them in a format appropriate to online use, follow these steps:

1. Select the image or images you want to export.
2. Choose Export form the File menu.
3. Choose the Web Page tab in the Export Photos dialog window.
4. Click the Export button.
5. Choose a place to save the images.

For complete control over your images, for detailed editing, for combining pictures, and for saving them in the most efficient format for your use, Adobe Photoshop Elements is by far the best tool. It's not free; you must purchase it from Adobe, but it is well worth the small cost. (And you can download a free trial version for Macintosh, and for Windows.

To work with an image in Photoshop Elements, first launch Photoshop elements, then use the Open under the File menu to find the image and open it. Once open, you may use all of Photoshop's tools to modify the image. When you are ready to save it, choose Save for Web from the file menu. Save the image in JPEG format, medium quality, and it will be suitable for online work.

Now you have a second copy of the images you need. Use these low-resolution copies for inserting into PowerPoint slides, embedding into web pages, or emailing to colleagues. They will work better, consume less memory, and allow quick downloading. You have mastered the megapixel magic of your digital camera.