Yes, with 10.8 now being the current version support for 10.5 was not considered a reasonable investment. Partly because of it's low market share (most have moved on to at least 10.6) and partly because v7 depends/benefits off of newer technologies like Open CL which (when it's working and not causing a crash) is a big boon to speed and responsively. OpenCL, in the form Phase leverages, was not around in the days of 10.5.
My solution is pretty easy! Assuming you have at least 40ish gb free on your hard drive...
Make a backup of your system as-is (just in case). I suggest
Super Duper.
Borrow/beg/steal a laptop with 10.7 or 10.8 and a firewire port (or thunderbolt port with a firewire adapter) and boot your iMac holding T
(firewire target disk mode).
On the 10.7 laptop use Disk Utility to create a new (empty) partition on your hard drive.
Install 10.7 or 10.8 (I'd suggest 10.7) on the new empty partition. It will not effect your 10.5.8 partition whatsoever.
You now have a 10.5.8 partition and a 10.7.x partition. You can choose which to boot into by holding option when you start the system. Regardless of which you boot into you can read/write from either.
You can then install v7 and be on your merry way. The only potential issue with this setup is permissions; so I suggest you move your photos to the "shared" folder in your user
s folder where there are no permission restrictions.
Unless the reason you can't upgrade is because you have a Power PC iMac (G4/G5) system. In which case: no, you'll need a new computer (little if anything in the last few years, OS or software, works on Power PC based machines).