The entire image is 178 high, and 575 wide.

Each costume is 178 high and 115 wide.

How do we identify an individual costume within the entire image?

Well, the costumes are always 178 high and 115 wide. We know because that's how we drew them.

We also know each costume is always at y=0 within the image, because that's how we decided to do it.

Here's the good bit: to find the x-position of where the costume starts, we just multiply the costume number by 115.

Costume zero is at x=0
Costume one is at x=115
Costume two is at x=230
etc