I’d be curious what others think but I’ve tinkered with Aseprite and it looks pretty great. I’ve only used the Trial, which doesn’t support saving, but it has a nice workflow/UI that supports animations and layers. It also appears to save Game Frame-style enumerated files if you create an animation and save it as BMP with the filename 0 (zero).
I’m not sure if the exports 24- or 32-bit files, but you can always convert. Plus I just released new firmware for Game Frame V2 that supports 32-bit BMP files on GitHub.