If you select the image, go to the inspector and click on the "Widget" (folder with a gear in the middle) tab you can add a "frame" by selecting one or all of the check boxes (title, caption, background) and you end up with a photo which will expand to full screen when tapped on the iPad.
It works great and if you don't need a title or a caption, just leave the text box empty.
However! There does appear to be a problem if you set up one of these frames in your template. For now, when I drag and drop a new image over the "framed" placeholder, the new image appears just fine. But, when you open the eBook on the iPad, the full-screen images all come up as the original placeholder image.
I haven't found a way to fix this, so as it stand right now I will have to layout each of these pages by hand. Not the end of the world for the size document I'm working on, but one **** of a job for anyone with a large batch of images to work with.
Hopefully someone will come up with the hidden attribute that needs changing in order to fix this.