and sorry to reply this late
No probs lol. I think saving/exporting would be a must have, but I also have two more suggestions that would make the program a powerful tool:
a - The ability to export a png of just a selected area (this would require adding a selection tool obviously).
b - Layers with opacity sliders. This may be harder to implement (or not, I really don't know) but it would allow users to rotoscope super easily. Then they could export each layer to a png and use something like Krita/Aseprite to touch up/add colour and pack to a sprite sheet.
I don't know much about creating apps, but would it be a solution to have an 'export' button that makes a checkerboard alpha transparency layer that automatically activates underneath all the other layers at 100% opacity, then you click on a 'save as' button which opens the file directory to save as png, bmp etc?
I'm only new to this whole coding thing so I don't know what's easier/harder depending on what language you're using and what lines of code you'd have to change.