I had a project in mind for pset0 but can't find the animal I want. How can I add a new sprite to my Scratch project?
you can always get a picture of one and import it as a sprite.
I designed a couple of the sprites in mine, search for the picture you want, download it, and then in Scratch go to 'Sprites:' at the bottom, you will see at the top-right of that panel there is a little face, a brush, a folder, and a camera. Those allow you to: select a sprite from the library, paint a sprite, upload a picture as a sprite or take a photo as a sprite respectively.