I thought about whether or not the spell would create an image of the mount as well, and could see an arguments either way.
On the 'Yes' side, I thought of several:
1. Equipment, etc. is also reproduced, and is produced by the images as it is used by the caster.
2. The spell is powerful enough that environmental effects (such as area spells) that cause visible changes to the caster are reproduced in the images.
3. It just wouldn't be very tricksy at all if Manny's images were all floating a foot off the ground with legs in position for riding a mount, but no mount visible . . . Since the spell description also states that it makes all sorts of accomodations to keep viewers confused it seems (at least somewhat) logical that it could reporduce a mount for that same reason.