Double tap doesn't zoom images full screen

[nonsequito]

I've added inline pictures to a Pages file, then exported it as ePub.

In iBooks, the layout is correct, but double-tapping doesn't zoom the pic to full screen. Instead the photo remains the same size, but is surrounded by a white border that fills the screen. Double-tapping again takes it back to the standard view.

Max pic dimension is 1600 pixels; exported as 200 dpi JPGs from Aperture.

Double-tapping used to work for me, but I must have changed something.

Thanks for any advice.