So what you would do is go to
and then type creeper into the search bar, for example.
Then paint over the 'creeper' image to make it look like a mouse. Just paint the body of the mouse.
Then paint over the 'skull_creeper' image to make it look like a mouse head.
Then, There's no need to rename the image from 'creeper' or 'skull_creeper' to something else. That won't work.
All of the images you paint in your texture named 'my mods' will show up in your minecraft world on our server, when you run the texture 'my_mods'