I love it when you can reach every function of the program via the keyboard. Of course, it wouldn't hurt if some important functions were additionally controlled via a GUI. I think this is one of the most necessary tools. Exactly when several sprites were merged into one atlas.

Also here I could imagine a kind of tool tip that is displayed when hovering the mouse pointer over a sub image. With information which keys execute which functions.

Some sprite sheets have horizontal and vertical spaces between the frames. If this would also be considered when assembling or slicing the frames. That would be really great.

What I noticed - after selecting one or more frame(s) and then selecting the same frame again with the LMB. Another sub image set is generated. Is this the way it should be?

It would be more comfortable if the sub images could be selected not only from left/top to right/bottom. But also from right to left or bottom to top. It is good as it is. But it would feel smoother.

I like the way you programmed the selection tool. Very cool! And the new addition of zooming in and out of the animation was a nice touch too because the included dwarf sprites are really small. Even though I don't really work with sprite-sheets all that much it's quite a useful tool. Hopefully you can work on the UI that would replace or work alongside the F keys, now that the jam has finished and you have more time.