"I also tried setting up another layout with a larger container of the same image and pointing the button to that using a script, and it didn't work properly. "
This is what I would suggest, I use this method myself in my solution.
I show the images initially in a portal, then use Go to Related Record, to navigate to a new 'preview' layout showing the image as large as possible, This layout also allows prev / next navigation of the related images.
What do you mean when you say it didn't work properly ?
Thanks for your response SW.
What I mean by it "didn't work properly" is that after I set up the layout, when I viewed it in layout mode and with only one record in browse mode it worked fine, but as soon as I had more than one record the image disappeared and I couldn't see any of the records either. (perhaps I did something wrong in the setup) I didn't use a portal though - I just made the container with the picture in it a button and pointed it to the other layout. I'm fairly new to FMP, not completely familiar with all the neat tricks and stuff yet.
I got it to work - Did what you said, just created a new layout with the image larger and this time it worked. Must have been something wonky I did. Thanks for the advice!