i mostly do photo stuff on my macs. i use the 13" air to transmit from live locations to my agency and it is great--the 1.8 ghz I7 works fine for photoshop including the actions i run. however, editing a large number of photos gets pretty tiring.
if i have a bigger job, i prefer as big of a screen as i can get. for instance editing on my macbook pro 17" is WAY easier than on the 13" air. so i would strongly recommend a 15" as the minimum size and would try to get a high res screen (either glossy or matte). most of the wire agency guys i work with use 15" models with the majority preferring the matte hi-res models.
you can go for the lowest level processor since any of the new processors are plenty fast enough. get an external drive to back up the images immediately.
if i have a bigger job, i prefer as big of a screen as i can get. for instance editing on my macbook pro 17" is WAY easier than on the 13" air. so i would strongly recommend a 15" as the minimum size and would try to get a high res screen (either glossy or matte). most of the wire agency guys i work with use 15" models with the majority preferring the matte hi-res models.
you can go for the lowest level processor since any of the new processors are plenty fast enough. get an external drive to back up the images immediately.