Hi ELIZABETH F.T. ,
The idea is that you have a custom visualization of the pop-up window (sorry for the Spanish interface)
In the configuration window, activate the HTML mode and then put your html code for the image (and all other info). Inside the source parameter "src" of the image tag you reference your expression "{expression/expr0}":
.
Your expression should be the part where a URL is generated:
The resulting URL is based on some logic derived from 1 or more attributes.
Can you explain a little more what defines which of the 15 images should be picked in your case?