Created attachment 6963 patch I wrote a patch that allows images to be flipped horizontally. I tried to follow the conventions of the existing code. The accelerators for the menu entries (C-{ and C-}) are oriented on the accelerators for rotation (C-[ and C-]). There might be better fitting or standard accelerators that I do not know off.
Thanks a lot! Merged as https://git.xfce.org/apps/ristretto/commit/?id=8de70164be5df55735aa3c098f96a7a4f5849af5