If you're working with photos a netbook is out automatically. Their screens are tiny and low resolution.
In fact, if you're working with photos, you should really be using a 17" notebook (some of the largest notebooks on the market) so you can actually see what you're working on.
Essentially, you should look for something that's got some config like this:
At least 1680x1050 screen, if not 1920x1200. (15.4" is probably for you, but 1920x1200 at that size will be tiny, tiny pixels.)
4GB of RAM.
At least a dual core 2.4GHz processor. Intel preferred (better performance, better battery life).
A decently large hard disk, 250GB+.