You're almost there, just need to unravel the 3 bits:
1 - SQUARE FRAMES - is the most basic form. Frame height equals width, Reaper just divides it all up, this is what it automatically does unless you tell it otherwise, which you would do using...
2 - PINK LINES - which are what you use to tell Reaper the height of a single frame, if its not the same as the width.
3 - YELLOW LINES - Here's what you use to do your hitbox. Anything outside the yellow area is treated as 'just some graphics', the bit inside is the interactive knobby bit.
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Don't forget to get horribly tripped up by how this is all very different to how pink and yellow lines are used on static images