I also had a similar problem today. Entered a few photos in FTMM2, synced, and what appeared in the linked Ancestry.com tree in each case was a placeholder image. Interestingly, I then attempted to upload the photos directly into the Ancestry.com tree, and they linked back down to FTMM2 correctly. I also deleted the placeholder images from the online linked tree.
Now nervous about adding any new pictures on the Mac but hoping this is a temporary syncing issue.