Masking Images doesn't work for Inline Wrap
A bug, I think ...
When images are set with an inline wrap, masking doesn't work.
It is necessary to set wrapping to, for example, "Above & Below" - the mask can then be edited/adjusted.
The wrap then needs to be set back to inline wrap and the masked image is now available.
Pages 5.5.3
iMac, OS X Yosemite (10.10.1), iMac (20-inch, Early 2008)