If you are using America Online, you probably have AOL's "image compression" feature turned on; it is turned on by default. A low-cost ISP, "NetZero," may be using similar software. This accelerates your browsing experience, particularly if you are stuck behind a 56K modem, by converting all or most images on the web into a proprietary format known as an "ART" image. The catch is that photos will sometimes look terrible with this compression.

 
To turn it off, first make sure you are signed on to AOL properly, not as a guest. Then, pick "members" from the toolbar, pick "my AOL," locate the "web" preferences, and turn off "use compressed graphics." For more complete information, see this excellent "before and after" comparison.