Try this array formula**.

Use cells to hold the criteria:

A2 = Photo
B2 = July 2013
C2 = Aug 2013


=AVERAGE(IF(U2:U100=A2,IF((D2:D100=B2)+(D2:D100=C2),AI2:AI100)))

** array formulas need to be entered using the key
combination of CTRL,SHIFT,ENTER (not just ENTER).
Hold down both the CTRL key and the SHIFT key
then hit ENTER.

You should avoid using entire columns as range references with array formulas.