Nothing wrong with using istockphoto, as a professional designer, I find there images can be just as good as getty, and they are less recognizable because they have only been used dozens of times rather than hundreds of times like gettys images.
As for Casale, I’ve never heard of them, but I would be wary of tossing $5,000 at something without knowing how well it would convert. If you have been successful at converting traffic thru AdEngage at a much lower cost, then why switch? Overall, I’ve seen PPC be mostly worthless, but have only tried smaller campaigns to test conversions, there is a ton of click fraud out there.