And there's less upgrading can be done with Apple stuff these days after purchase. My Macbook seemed fast enough with 2Gb ram and a conventional hard drive when I bought it 5 years ago, but now with 4Gb and an SSD it struggles to do image editing. Ask yourself what you want to be doing with it in 5 years time, and whether the upgrades you might spec now will be enough to keep up that far in the future (I'd suggest they won't be, but YMMV).
Adobe style image editing software seems to require a big screen - I find a 20" 1650:1080 monitor at home slightly cramped, 24" 1920:1200 at work is better - so ask yourself what you're happiest working with.
*Personally* since you seem sold on a Mac, I'd suggest a slightly upgraded Mac mini plus a high quality after market monitor. That way you'll probably save money AND have a decent monitor that is likely to have a longer working like than the iMac, plus if you find the MM becoming limiting it can be flogged & replaced for half the cost of the iMac. I do understand the appeal of having everything in a single unit, no extra boxes, cables etc, but it seems an limiting way to buy a laptop & screen.