Yes, a lot of questions with no clear answers. And I'd be hard pressed to give you a straight answer to any of them.
What kind of photo editing do you do? Is that your source of income? Is your lively-hood dependent on 'Photoshop'?
If you don't do advanced photo editing but rather just lighten photos, remove red eye, brush out the jackass who bombed your romantic photo by the waterfall, etc.... then yeah, GIMP should be a snap. The only 'big' change for someone like that would be finding replacements for their favorite macros and such. After all, a brush is a brush, a pen is a pen, and a plate of pad se ew is freaking' divine!!
Not knowing your Mac I can't suggest you just VirtualBox the Linux install and play from there, it might not be able to handle the overhead. But I'm about as sure that you can dual boot as I am sure that my feet smell like boiled cabbage after a long hike.
Truth is, I like Macs. They underlying code is pulled from *BSD. You get yourself a shell on that puppy (Terminal) and you can really have some fun. (I have WindowMaker installed on my MBP)
But I'm a FreeBSD fan, typing this post using it actually.
Options options options my friend. And I guarantee you that no matter what you chose now, you'll end up going to something else. That's the nature of the bug. You switch and change and try distro after distro until you find the the one that puts the olive in your martini, the relish on your hotdog, the saltpeter on your....... ouch, anyhow.
You can always give ElementaryOS a try. It'll feel just like home to you.
Or, and here is my advice, find the distro that has the best documentation for getting it to run natively on your Mac, with all the bells and whistles functioning. And if you have to ask yourself "can I get this to run in WINE", then maybe you should ask yourself "do I really want to run Linux"....
Just a thought.