Getting the max height from the img files correctly (Bug #1266586)

Problem existed where the cache didn't know the height of the files as
they had not been loaded before so the getting of the hightest one was
failing.

Changed it so that mahara passes in the height of the file to allow it
to fetch that instead.

Change-Id: I0a2cd8d316544cce65da73c1a7787759d5d6706d
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
Merge "Getting the max height from the img files correctly (Bug #1266586)"