First of all you do not need make images if you don't want to. It's not mandatory. Just edit protected/modules/badger/views/_list.php and remove part that pointing to image. So you can make your badge style as you want.
But if you ready to create images there is nothing complicated
Edit resource files
Resource files can be found in badger/res/
Save new images into badger assets
Export new badge images into badger/assets/images/. And don't forget to name image files as badge slug (inserted into badge table slug column, and constant value).