Here is how you will need to set this up:
Create an e-mail template by going to Settings and clicking on Email Templates. You can also start typing "Email Templates" in the Global Search box.
You can either Create a new template or use an existing one by making a copy and changing the name. This allows you to use pictures if you already have a nice looking template.
When you create your Season, you have the option to include a Registration Email if you like at the Season Edit Page or you can also choose to select a different one at the Level Edit Page.
Season Edit Page:
Level Edit Page:
When you add a customer to a roster of a class, camp, or team you will see a check box that is checked by default. If checked, the registration email assigned at the season level (or Level page) will be sent to the customer when they are added to the roster.
Here's a shot:
