@James – Hm, I found no way other than animate the graphic frame holding the image.
First I thought, I'd test this with a graphic frame pasted inside another graphic frame, but then the animations options are grayed out as well; just when you select the image instead the frame holding the image.
To mask everything around the animated graphic frame, you could use a rectangle filled with "[Paper]" bigger than the spread and punch a hole in it sized and positioned exactly like your right page showing the image. You can do this with the Pathfinder Tools (substract object behind). Put that mask on a layer above the animated graphic frame.
Ok. What will work somehow is:
Have a duplicate of your image sized like you wish where the bounds of the graphic are the bounds of the graphic frame.
1. Set the animation as you like.
2. Position the animated frame on the rectangle currently holding your image.
2. Then cut/paste into the animated frame into that frame.
Test the animation in the EPUB Interactivity Preview Panel.
No need for an extra mask on an extra layer then.
Here two screenshots showing two different set-ups for the same image.
1. Masked image selected => Will rotate under the mask
Animation Panel is showing the animation settings.
2. A duplicate of that image pasted inside a square => Will rotate inside the square
Animation Panel is not showing the animation settings.
The EPUB Interactivity Preview Panel is showing both images rotating side by side.
UI is German.
Woohoo! Yes! That worked! One note for anyone else doing this you have to do a "Paste Into" rather than a regular paste. Otherwise, works like a champ! Thanks so much!