There is no such button for uploading images. You'll need to upload them to wiki repository that I've mentioned above.
Basically you need to:
1) clone this repository
https://wiki.prs-plus.googlecode.com/hg to whatever folder you like
2) put your images there
3) "add" those files (should be possible via right click -> some toroise menu)
4) "commit" the changes
5) last, but not least: PUSH the changes to
https://wiki.prs-plus.googlecode.com/hg.