Wiki's are a little backwards when it comes to images.
Before you upload the image, you have to pretend like it already exists, and add its code to the page:
Code:
[[File:YourFile.png]]
Then, when you save (or just preview your edit), you'll see a red link on the page where you image should be. Click that link, and you'll be taken to a page where you can upload your actual image (and after you do so, when you go back to the first page your image will show up).
See also:
https://wiki.project1999.com/Editing