2 solutions!
I can't stand ANY of the mac softwares for image editing - APART from this one i just found which i think is AMaZING!
(and xee which LOVES to crash)
Imaginator by stone - it's not free - but has a great trial version and it's hugely powerful
failing that I always go for installing a windows copy on your mac and then using irfanview which desperately needs a mac developer to convert it's OS base.