I've improved the modal popup for Related Image Gallery as suggested.
It now has a previous/next/close button (and you can use left and right keys to navigate, escape to close, or click anywhere on the page to close).
It also centres the image in the modal, and resizes if the page is too short for the image.
And finally, you can choose whether to show a caption on the images.
As mentioned in my other post, I've created a Photo Album includelet which works in the same way, but takes a Photo Album as the source for the images, rather than images related to the page.