This is one that a lot of modals get wrong (but if you look at any major site, they get it right).

If the content of the modal is too large to fit on screen, users should be able to scroll the modal content (especially images). I settled on this plugin but this usability issue is a show-stopper.

You can test this by making the browser really small on the demo page and loading a modal which doesn't fit. Then try scrolling.