Thanks for your response Ariba, but I'm still confused. That tutorial shows you how to add a modal popup using the Modal Popup Addon, not the Image Layout Addon. The blog article I linked to above suggests that you can add a popup on the Image Layour Addon - can you confirm if this is actually possible or not?
Here's the full text from the "Image Layout addon brings grid control" section of the blog article in case it helps:
Image Layout addon brings grid control
The Image Layout addon comes with six astonishing image layouts. Every single one of them has individual characteristics to their names. One of the card layouts has got column controller (shown in the image below) option, which means you can allocate how many columns (grids) you want to allow for the image, the rest of the columns (out of 12) will be adjusted automatically for the text content. In addition to that, you can now add video popup (modal) in this addon when the same card layout is selected. When video popup is enabled you’ll be given a field for placing the URL.
Thanks...