Yeah, it's impossible to just rip the graphics and have them work like that. The SMW Rex is 16x32, while these graphics are 24x32. You'll need to request a custom sprite.
Post your request in the
ASM request thread and be specific. You need a 24x32 Rex sprite that flattens to 24x16. Choose how many frames you want to use for normal and flattened states, because your image has 15 normal frames and 10 flattened frames, which is
way too many.
In the future when making a thread, please check to see whether or not a thread already exists for that purpose.