Wikipedia's goal is to be a free encyclopedia. That not only means that it doesn't cost money to access; it also means that people are free to reuse its content in their own works, even commercial ones, so long as they provide credit and release any changes they make under the same free license. Therefore, the images used in Wikipedia ought to be free for other people to use. Wikipedia does not generally accept images found on the Internet, as the vast majority of them are not freely licensed.
Free images must allow:
If you are contributing an image which is your own work, the simplest option is to release it under the Creative Commons Attribution-ShareAlike license. This is the same license used for text contributions to Wikipedia, and requires reusers to credit the image to you (attribution). You may also choose another free license from the list on the upload form. Be careful, though, if your image includes another person's copyrighted work (other than incidentally) then you do not have the right to freely license it.
You may also upload images which you know and can prove are under a free license or in the public domain (e.g. due to age).