(This is a template, feel free to delete it.)
I have no idea how to use snap and I don't know how to use it What's problem you are encountering?
snap's ui is terrible What have you tried that didn't work?
what actually are you trying to do that you can't figure out how to do? "using snap" could mean doing all sorts of different things, and people here can't explain how to do every possible thing in snap
You can import costumes by dragging and dropping images into snap, using file > import, or file > costumes. You can then switch to the costume to not use the turtle sprite. Of course I do need to mention, you cannot get rid of the turtle costume, because that is the base sprite without any costumes.