Clone it to your local box, create pages, add images and check them in. The push them.
There may be online tools for that too … but see the simple Login page I just added.
tony@dragonfire:/opt/gitrepos/LibreEHR.wiki (master)$ gitlog
0f3a610 Add images dir and Login Image
A images/LibreEHR-login.png
20113ea Updated Logging In (markdown)
M Logging-In.md
080f15f Created Logging In (markdown)
A Logging-In.md
ffb35dc Updated Patient Flow Board (markdown => mediawiki)
D Patient-Flow-Board.md
A Patient-Flow-Board.mediawiki