Simple image-only Popup proving difficult

I’m a newbie user of Popup Maker using version 1.2.2 with Elvrye theme I’m trying to implement a simple popup of a single image to a larger image, as per the example here http://www.exodusmediasales.co.uk/market%20info.htm – the popups for the round and house graphs.

No matter what settings I select the result is that the expanded view is in full screen, with no option to remove the display other than back-page.

I have successfully created popups that work successfully but these contain a mixture of text and images.

Am I missing something simple here or is what I’m trying to do not possible?

https://wordpress.org/plugins/popup-maker/