Wikipedia:Image use policy
NOTE: As of July, 2002, this is an early work in progress. This should be seen, then, as a proposed policy, subject to dicussion and change.
Images are for illustrating articles
The primary and most important policy is this: the purpose of the upload facilities of Wikipedia is to allow authors to upload illustrations to accompany articles. Don't upload images just because they're interesting, or because they might be useful in an article. Just upload ones that will actually be used.
Use appropriate format and size
There are many technical hints in this section that some people may not have the tools or expertise to deal with themselves. If, for example, you find a great image that needs to be resized or recoded and you don't know how to do that, ask someone on the Wikipedia-L list to do it for you.
Scale and crop images to a size appropriate for the article. Keep in mind that many readers are using 800x600 displays, and so images wider than 300-400 pixels may overwhelm the article. Larger images also take more time to download over slow links. Likewise, images smaller than 100 pixels may be difficult for users of larger displays to see. Don't use tiny "thumbnail" images linked to a large image--use an image of the appropriate size; adding a link to a larger version (perhaps the original source) is fine as well, but don't upload the larger one unless it is really needed.
Drawings, icons, political maps, and other such images with limited colors should be in PNG format, preferably grayscale or indexed color with fewer than 200 colors. GIF images should be converted to PNG before upload unless they are "animated" GIFs. Do not use PNG for "photographic" images, unless that (or GIF) is the only format available.
Photographic images should be in JPEG format, with quality settings set to make a reasonably sized file. Do not use JPEG for iconic images or maps (except possibly for photo-like maps that show terrain and such), but if you find an original in JPEG format, do not convert it to PNG before upload. In particular, the maps from the CIA Factbook website are incorrectly coded as JPEG. If you have the tools and knowledge to do so, render the original PDF maps at high resolution, then rescale them and convert to indexed PNG. The result will be both a higher quality image and a smaller file than the JPEGs from their site. But if you can't do that, then go ahead and use the original JPEG. Try to avoid cropping or otherwise editing JPEGs too frequently--each edit creates more loss of quality. If you can, find an original in 24-bit PNG, edit that, and save as JPEG.