Question about the "image size" and the "transform" tools

I am doing app reskinning. The trick with it is that the original and new art need to be the same size. Do I need to just change the image size? Or do also need to have the "transform" pixels the same as the orginial? Maybe a better way to word my question is, are the "image size" tool and the "transform" tool the same thing? Because whenever I change the image size, my new art stays the same. But when I change the pixels to match the original in the "transform" tool, my image distorts. Can anyone clear this up for me? Thanks!