From The MegaTF/TeamFortress Wiki!
Click on 'Upload file' on the sidebar to the left under the category Tools.
Here is a direct link if you've changed your view/template and cant see it anymore: Special:Upload
Once there, choose the file, fill out any fields if you wish and hit upload.
You can reference your image by using the tag: [[File:filename.ext]].
Pat yourself on the back for uploading your first image!