Like Henri already said, you really can't use globals for anih, ani_frames, and ani_frame_idx. These should be in a structure, and decodeRIFF should write into that structure, not into globals. The structure should then be associated with the icon.
They're temps.. I've added a comment to make that clear.
That doesn't make it better, it's not thread-safe. Pass them as parameters to decodeRIFF.
This may be obvious to other, but certainly not to me: what does 0x00030000 mean? You need to use symbolic constants here.
It's the version non-sense that Microsoft's api requires. You'll find that every call to this api and it's non-Ex variant have it.
I still say a symbolic constant is better, or at the least, a comment. A test case that shows that it's needed is even better. --Juan