I feel like you're looking for a lazy way out, when you should be formatting your character sheets the correct way and not be so lazy.
Find the largest ass image you have in all 12 frames, then format the rest of the frames to the same dimensions as that frame.
IE: If frame 5 is 40x25 PX, then make every single frame equal 40x25 PX. Then align each image inside the frames correctly so that you're left with a smooth animation.
I don't know if you know this or not, I'm assuming you don't, but as long as every frame is the same dimensions, the program will cut it up evenly into 3 columns and 4 rows (given that you name the file with a $ at the beginning). This means you can have basically any frame size as long as they're all the same.