The free pixel animation editor Piksel has been updated to version 12 which has some really great features, especially a crop feature for animations.
If you find great animation sheets on Tumblr or others, you can load the GIF into Piskel, Crop to a 16×16 part and, if the GIF has been zoomed (like many) do the resizing of a 48×48 with 3x pixels into a 16×16.
Piskel can export into a ZIP file with single GIF images or into a GIF animation. I take the ZIP, convert to BMP, tweak a config.ini and ready!
I was looking long and hard for a crop tool that works with GIF animations on EVERY frame and Piskel does the job very well – and for free!