As you mentioned templates, I'm assuming you are working on a Trailer in iMovie (not a standard project). Unfortunately, the templates accept video only - not photos or stills.
A way around this is to convert the Trailer to a standard project. Then you will be able to add stills, and perform other edits as desired.
To convert, select "File > Convert to Project" from the menu. Before doing this, it's best to duplicate the Trailer so that all your original edits are intact in the Trailer format. To duplicate, while in Project Library view, click on the Trailer name then select "File > Duplicate Project" from the menu.