21.5" vs. 27" iMac for Photo Editing

Hi guys, if anyone out there has a little advice for me on buying an iMac, I would greatly appreciate it if you could just take a little time and help me out.
I'll kind of explain my situation here so you guys can get a little background on my budget and such.
I'm a 17 year old Homeschool student. I'm going to be using this iMac mainly for photo editing and just taking some notes for school, with some occasional video editing every once and awhile. I'll also be making some music on it every once and awhile and connecting a MIDI keyboard to it.
I'm going to be paying for this computer by myself. I mean, I don't really need to specify that I'll be paying for all of this. For some reason though other kids now and days seem to get whatever the hell they want from their parents, so I wanted to make sure you guys understand that I'm going to be working hard to pay for this myself, just like everything else that I own, just so that people don't assume I'm some spoiled kid.

Now, I have a job that pays alright, considering I don't have too many expenses to pay for, also considering I'm 17. I work a couple days a week and am currently saving up to afford this computer. I had a few hundred saved, except today I just had to use that to pay my income taxes for the year... woohoo.
I currently have an HP DV7 laptop that I'm going to be selling to afford this iMac. It has a 17" screen which is big, for a laptop. Except when editing photos on it, even though I've calibrated it as best as I could, there is a big difference between prints and what I've been editing. Also even though the screen is large for a laptop, editing a few hundred RAW files on it, and trying to keep an organized workflow gets a little tricky/annoying.
So those are my reasons for upgrading from my laptop.

I've read a few different people's opinions on which iMac is best for photo editing. Some people say go with the 21.5" and buy an external monitor right away. Others say go with the 27" and buy an external monitor if you need it. I just don't know which one would be right for me. So that's why I'm hoping someone who has experience can give me some recommendations on which iMac to purchase. Especially since my budget is basically the cheapest option I can get that will be the best for what I'm doing.

Also, I just want to add, please don't turn this into a Mac vs. PC debate. I've made up my mind on purchasing an apple computer and even though it's more expensive, I'm going to wait and save up more for one. Maybe I'm just ignorant, but it's my choice to wait and save up.

So hopefully I can save up enough for whichever option I choose soon. I'm selling my DV7, drumset, possibly xbox, and anything else I can sell to afford this. I've even considered giving up dirtbiking to afford this computer. If you're interested in anything, let me know. Haha.

Thanks so much for any help guys. I'm sorry for the long read too. I tend to go off on little sidetracked stories when I make threads. Sorry about that.

I searchbarred too and didn't see anything on this type of situation.
Thanks again.