One of Wikipedia's "five pillars" or basic principles is that Wikipedia is free content, including text and images. "Free content" doesn't just mean that it's available without cost. Beyond that, it allows people to use it for any purpose with few restrictions. The exact definition that Wikipedia uses is found at http://freedomdefined.org/Definition/1.0.
Images generally have to be free content. Images that are free can be uploaded and used anywhere on Wikipedia. Images that are non-free (not free content) can be uploaded and used, but only under certain conditions.
The first step before uploading an image is determining if it is free or non-free.
