Great news!
Besides fixing a few minor bugs, I added a new feature to the app, the "Manual Crop". You can access it in the FX tab, by clicking on the triple dot "..." button next to the "Resize button".
I basically allows you to  manually cut out an area from an animated image or animated spritesheet.


For example you can extract the walk animation out of this animation with it:
Turn this:

To this:

And if you want you can afterwards combine each extracted sprite sheet:

If I find the time, I will do a quick tutorial on it

Full List of Changes:
-Added "Manual Crop" feature
-Added "visualize horizontal sprite sheet" and "visualize vertical sprite sheet" buttons, so you can directly in the editor arrange each sprite in one line (The buttons can be found in the Export Section, with an eye symbol on each button)
Bug Fixes:
-Grid for previous image remained active when importing new image, now it deactivates
-Importing gif or apng images that are static  with only one frame, were not imported correctly and couldn't be cropped as a sprite sheet. Now they can.
-When moving the rectangle on the grid, moving the upper boundary would mess up the rectangle if the lower boundary has been moved as well, now is fixed.


Thank you for reading, and I hope you find the new features useful!
-Lion In A Box