It would be nice if you could just right-click an image (or use the File menu) and choose an "Open with ..." submenu entry in order to open the image with any application you like. The entries of this submenu should be editable. I prefer the way emelFM2 does this: if you right-click a file and choose "Edit filetype.." you can specify both a label and a command line to run for every "Open with..." submenu entry.
This way, you could provide a workaround for the lack of support of animated GIFs: The user could just open it with "mplayer -loop 0 %f", for example.
And, of course, thank you for all the work you have done so far!