You can manage your folders/clients in the account menu on the left.
Expand the menu to add an additional folder/client. Or, click on your account menu option at the top right.
At the very bottom of the page, click '+Add Sub Account' on the right.
Fill in the sub account information:
The Title you chose will be select-able from the left hand menu.
Click on it to switch your dashboard to your 'Sub-Account.'
Once you are switched over to one of your sub accounts, it will have its own image templates, message templates, campaigns, order history, and settings, which include default backgrounds and return addresses.