Is there an Air Video app, but for photos?
I developed ImageBank for iOS to do exactly this - streams photos from your PC/Mac to your iPhone, iPad or iPod Touch.  It's universal so it works natively on the iPad, and it even works over 3G.

There's a free version too if you want to try it out:
ImageBank Lite

It's only US $0.99 to buy or upgrade from the free version.