In Scratch the charecters are called Sprites. You can paint your own, upload one from the scratch library, take a picture or upload your own one.
So, when you click on the upload button, you can upload your own sprites. If you click on the camera button, you can take a photo. If you click on the paintbrush, you can paint a sprite and if you click on the choose sprite from library, you can upload a sprite from the library. Have a look at my video all about adding sprites and how to draw your own!