Occasionally, having a free image for a particular subject may not be possible. Therefore, the English Wikipedia allows a limited amount of non-free content in specific circumstances. Examples include cover art for books, music and video games, corporate logos, and screenshots of web pages or software. The legal basis for this is the US concept of fair use, although in fact Wikipedia's criteria are stricter than this to keep the amount of non-free content to a minimum.
The full requirements for non-free content can be found at Wikipedia:Non-free content criteria. In summary, a non-free image may be kept only if all of these conditions are met:
It is used in at least one article. (Use on non-article pages, such as personal user pages, is prohibited.)
Its presence in an article significantly increases readers' understanding of the topic, and omitting it would be detrimental to that understanding.
No free equivalent is available, or could be created, that would serve the same encyclopedic purpose.
Its use does not negatively affect the commercial interests of its owner. (This usually means that the resolution is not higher than needed for the purposes of understanding).
The image must also have on its description page:
Information about the source of the image, and if possible details of the artist, publisher and copyright holder.
The easiest method of uploading a non-free image is Wikipedia's File Upload Wizard, which will help you add all the required information. A link to the File Upload Wizard can be found under "Tools" at the left of the screen.