Although I'm not sure the best way to explain how to create the animation, I have a feeling you're over thinking this and expecting GIMP to automatically do some things for you. An animation is just a series of images. The series of images for breathing would start with your basic sprite standing there followed by frames where the sprite's body, arms, and head (but not legs) are a little lower than they are in the first frame until the point where you draw frames to transition to the first frame again.

You could just select the body and everything above the legs and move it down a pixel so that it's now over top of one pixel's worth of the legs for the second frame. Depending on your sprite, it might look a bit strange and you may have to edit it further to get it to look right. I don't know if GIMP has more advanced tools that'd make editing easier (I haven't really used it before) but that's the way I'd do it.

How do you do that?I use the selection tool+move.But it moves the sprites,how do you only move chest?

How do you do that?I use the selection tool+move.But it moves the sprites,how do you only move chest?

I've never used GIMP before, but in Photoshop you would use the selection tool then just copy/paste, and it pastes the selected region into a new layer. Once it's in a new layer, you should be able to move that layer independently. I can't imagine GIMP would do it any different, since copy and paste (Ctrl+C and Ctrl+V, respectively) are a staple.