Convert costumes to a list! From that list, you can load a costume!
https://snap.berkeley.edu/snap/snap.html#present:Username=joecooldoo&ProjectName=Pen%20Costumes
I'm a little confused. Why is this better than just straightforwardly using the costume itself? You can use the 
block to get at the pieces of the costume, including its pixels.
If you attempt to switch to a costume with that block, well, it comes out messed up.
Oh, you switch to a costume just by putting the costume itself (not its pixels) in the input slot of SWITCH TO COSTUME. The __ OF COSTUME __ block is useful only if for some reason you need to examine the innards of the costume.
Exactly. You can find this useful in drawing projects.
that's because you're not defining the width and height. You'd have to use the new costume block to make it not messed up.