Uploading files to Wikipedia can be a bit of a chore, which is why the File Upload Wizard was developed. It is a series of pages with options to choose a file from your computer, provide a name and information, and identify the correct copyright.
It's pretty straightforward—at least until you get to the copyright portion. Because Wikipedia has very strict limits on what kinds of image and file licenses it can accept, if you are thinking about uploading a copyrighted image, the Wizard is going to ask you for a lot of justification, and you'd better have a good answer.
There is also an Upload Wizard for Wikimedia Commons, the Wikipedia sister project and repository for all freely-licensed images. Or if you like to kick it old school, you can always use the original version.