Sketchy has uploaded a very useful article for people pondering how to import many frames of animation into MMF2 without having to resort to the gif format, which is not suited for pixel art at all! It’s really worth a read and keeping in mind for future projects.
Comments from Sketchy: MMF2's picture editor is very limited compared to almost any dedicated graphics app.
However, if you use another app (such as Photoshop), you are left with the problem of how to load your graphics into MMF2 when you're done - which can be an extremely tedious task if you have many frames of animation.
Fortunately, MMF2 includes a neat feature which allows you load many frames of animation from a single spritesheet image, provided it is properly formatted.
Click here to read Sketchy’s article