Q. When I pull up an image from a CD it is nice and clear, but when I download an image from the Internet it just doesn't look as good. Why the difference?

A. If you go camping with a truck, you can throw in a Coleman portable stove. Backpackers, however, are famous for being very careful about how much weight is in their packs; they are the ones who have to drag the things up and down the trail, so they economize. There are parallels in the computer world.

A typical CD can hold over 600 megabytes of data--you can load it up with very large and detailed graphic images. But Webmeisters are like our backpacker friends, careful about how big an image they have on their sites. Generally speaking, the images they offer up are only at the relatively low resolution of 72 dots per inch. That is good enough to look at on a typical monitor, but not terrific.

Sites limit themselves to this resolution for reasons of speed and storage. Smaller images transfer faster on the Internet. And many Web sites pay their service providers by the amount of space they use. A site may be limited to 10 megabytes, for instance. So they get very stingy about the size of graphic files.

Mom has an iMac and AOL; I use a Windows computer and a regular Internet service provider. What is the best way for me to e-mail her pictures?

Graphic images take up much more space in a computer than good old text e-mail. So for years computer users have been compressing these graphic images before sending them over the Internet. Math majors will argue for hours over how this is best accomplished--the general reader must know only that there are several ways, or formats, to use to compress an image.

Our reader is concerned because he knows Apple's colorful iMac uses a different operating system than the one he uses. That's not a problem here. A Mac can handle a wide variety of formats, wider than what the standard Windows personal computer can handle.

So I'd say the solution here is to render pictures in the JPEG (Joint Photographic Experts Group) format--many photo-editing programs let you do that--and send them to Mom as attachments to e-mail. America Online's software for the Mac will have no trouble opening these files, which end in the extension .jpg.

If your mom needs more information on different file formats on America Online, she can type the keyword HELP. It will list many different kinds of files that AOL handles.

I was wondering if you are familiar with the Quark PDF files--when I try to make them, I keep getting the message "can't find Acrobat Distiller."

With the advent of laser printers, specialized programs called desktop-publishing software came into use to allow you to create great-looking pages. Engineers developed a technology called "PostScript" to make laser printers understand what they had to print. It became very popular.

With the advent of the Internet, a standardized way of presenting an image over the Net for a variety of software platforms (Windows, Mac, Unix, for instance) was required. A format for accomplishing this task called PDF (portable document format) became popular.

Two big players in the desktop-publishing software category are Adobe's Pagemaker and Quark's product QuarkXPress. Both programs treat the creation of PDF files like the Texas two-step. The program first generates a PostScript file. From there, both programs give that file to a product called Acrobat Distiller to turn it into a PDF file.

This Distiller is part of the Adobe Acrobat product. Chances are that you don't have a copy of Distiller on your machine. You can download it from Adobe. For information, go to www.adobe.com/supportservice/custsupport/LIBRARY/acdwin.htm.

John Gilroy of Item Inc. is heard on WAMU-FM radio's "The Computer Guys" at 1 p.m. on the first Tuesday of the month. Send your questions to him in care of The Washington Post, 1150 15th St. NW, Washington, D.C. 20071-5302 or via e-mail at jgilroy@iteminc.com.