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.
Just like the articles, it is important that images follow the same guidelines about copyrights. Make sure you own the image, or that it is in the public domain, or that the copyright holder has agreed to license it under the GFDL.
Add good descriptions
Each uploaded file has as associated image description page into which you can put text, and which shows the image's history and usage. You should at least always include a brief description here, and especially mention where the image came from (perhaps with a link to the original source), and what its copyright status is.
Descriptive titles are also useful. Uploading a file named, for example, "Africa.png" is likely to collide with one already present, and doesn't give any clue about its contents. A more descriptive name like "Africa_map_2002.png" is better. Avoid special characters in filenames or excessively long filenames, though, as that might make it difficult for some users to download the files onto their machines.
Do not put image credits in the articles using the image. The image might be used from more than one place, and so such glosses might go out of sync. Also, the article might be edited, and we could lose information about the image itself. Finally, it puts irrelevant information in the article anyway. That information belongs on the image description page, which also enables you to make much longer descriptions without affecting any of the articles that use the image.
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 World 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.
Sounds should be encoded in Ogg Vorbis format.