Hi Colin,

Try this...

Place each thumbnail on your page using a separate Image Object for each one.  Be sure that the 'Display Mode' (listed on the Display tab in the Image Object properties) is set to 'Automatically resize image'

Then move to the 'General' tab in the Image Object properties.  Select 'File or URL with ShowBox' from the 'Link' drop-down menu  -  then set the path to use exactly the same image file.  You can adjust the size of the zoomed image using the 'ShowBox Properties' on the same screen.

You can check and tweak the results in preview mode...  no need to upload the project