To add a badge design to a credential group just following these steps:
1) Log into your Accredible account
2) Select 'Groups' from the top menu
3) Select the group you want to add the badge for
4) Select the "Group Settings" tab
6) Click 'Information and Appearance' in the left hand menu
7) Hit the 'Upload New Badge Image' button.
7) Hit save and you're done!
You will need to make sure that your badge is Mozilla compliant being 256x256 pixels or more, a Square image and PNG file.