Ah good point on passing a file to the appimage; I had assumed it was possible.
I believe you can put a .desktop file somewhere in ~/.config that’ll add it to the context menu. I’ll try with geeqie later tonight as it uses the .desktop file convention.
Edit: looks like “open with” depends on what file manager you use… Sigh… For Nemo, you can use the desktop convention, but it goes into a dir that needs root access… I’ll keep poking at this.