Only upload screenshots that have not been stretched, modified or distorted in any way. (Cropped screenshots are fine, as long as they're not stretched or distorted.)
Avoid including extra stuff in the screenshots, such as the play button of your player. (Try VLC or another player with a snapshot function!)
Try to get as high of a quality screenshot as you can manage. Use HD sources if you are able, and save in a lossless format (such as .png).
Look at forum #0004 for what kinds of tags you should add.
Do not upload drawings or vectors, even ones traced from screenshots. We can't be sure that everything is exactly as you would see in the screenshot.