As a photographer you'll want lots of storage. Unless you have an external drive you may want to consider replacing the HD with another HD (same vendor) as this model uses a custom cable harness for different HD vendor HD's. Here's a little write up on the issue Proprietary cable can put the brakes on upgrading Late ’09 iMacs.
While you may also want to improve the speed of your system with a SSD I wouldn't swap out the HD for the SSD. Instead, I would swap out the optical drive for the SSD. Follow this IFIXIT guide Installing iMac Intel 27" EMC 2309 and 2374 Dual Hard Drive
You don't need any software solution or 2.5" to 3.5" adapter frame here with this setup.
Are you planing on cloning your current internal HD? If you are clean things up and make sure to run Apples Disk Utility to make sure you don't have any errors and the permissions are OK. (You'll need to boot under a different drive to do this correctly).
Then using the external Lacie HD as the bootable drive and the SSD connected via a SATA to USB adapter cable clone your drive over. This will be very slow so go out and have a good time at a bar or student center... Once the drive is dup'ed swap it out. The problem here is everything is moved over which you likely don't want.
The other option is:
Swapout your internal HD with the SSD. Then just boot up with your external Lacie HD (double check before you start things) which you've already downloaded a copy of the OS installer onto (make a spare copy in a sub folder).
Once the OS is installed and you are booting up under the SSD you can then install your Apps afresh onto your SSD. If you use CarbonCopy you would end up getting everything moved over which you really don't want. Leave at least 1/3 (or 1/2 better) of free space on the SSD so the OS can cache effectively. To reuse your internal drive (as a backup) get your self an external FireWire case.