For Shield ANIs:

Must have a specific palette (FS-Shield-ANI-Palette.gpl)
Must be 112x93 in size
Must have four frames

For Head ANIs:

Must have a specific palette (FS-Head-ANI-Palette.gpl)
Must be 160x120 in size
No specific number of frames required

For CB ANIs:

Must be 440x200 in size
No palette restriction, though CB ANI must have a palette.
No specific number of frames required