Hi,
Is there a way to populate an entire template, say 8-wallets, with one image using just one click instead of eight clicks?
Yes but not quite one click if you count the preparation!
Open the template layout from Print Properties - Custom
Set the number of "Copies to Add" to 8, see screen shot below.
Select the thumbnail
Click the red + button adjacent to the Copies to Add box; NB. the + becomes red once thumbs are selected
That will load your template with the 8 copies of the one image. Done
Don't forget to set Copies to Add back to 1 when finished

Terry