All images shown on UOGuide need to be uploaded using the Upload file page. A link is located in the blue menu on the left side of every page.
The following image types are allowed: gif, jpg, jpeg and png. The type largely depends on the image size. If you have a relatively small image then gif or png is preferred because it will not compress or distort the image at all. If your image is large (50KB or bigger) than it is better to use jpg to keep the file size down. Also, jpg images make much better thumbnails than other types.
It is important to give the images a descriptive but concise name. For example, if you are uploading a picture of a fire ruby then calling it a "fire ruby" is best. Not something completely nondescript like "image12" or long like "red fire ruby in backpack". Image names should be all lower case (except first letter, UOGuide automatically upper cases it) and words separated by underscores. For example "Fire_ruby.gif" is correct but "FiRe_RuBy.gif", "Fireruby.gif" and "Fire_Ruby.gif" are not. Special characters should also be removed such as apostrophes.