I have had photoshop CS2 for many years and I got a new computer that is a Mac. I downloaded it but the computer won't install it... saying.... "can't be opened because it is from an unidentified developer."
CS2 will not install on modern Macs running any version of OS X later than 10.6.8. Today's Macs run 10.12 (Sierra).
There are no workarounds for getting CS2 to run on modern Macs.
The other problem wold be hardware. New iMacs are MacIntel boxes; CS2 was built for Motorola's PowerPC platform ( i.e., non-intel box ). There is a workaround. Just get a PowerPC based iMac and install on that.