to make it short and enlighten the occuring Leopard compatibility issues a bit :
10.5.x does not run "carbonized" applications ( apps designed to run under 9 and X ),
only pure Cocoa Applications and many of them might even need some sort of update, because in 10.5.x many APIs ( Systemfunctions,Frameworks ) have changed to make use of Animation,3D, Spotlight search and what not else.
Photoshop 7 and its counterpart for Homeusers, Elements 2.0 are both Carbon, while CS,CS2 and Elements 3(CS) and 4 (CS2) are Cocoa. CS3 is the first Universal Binary, running on Intel and PPC G4/G5.
What else might raise Questions : Imovie,idvd : Versions 1,2,3,4,5 are incompatible. Iphoto 1 and 2 too. idvd Beginning with 6.04 and 7.0.1 leopard is supported., iphoto as of 6.x and higher.
Apple Works 6.29 ( latest ) is incompatible because it is carbonized and is called obsolete/replaced by iwork ( at least 08 has to work under Leopard ).
Maybe somebody wants to put up a list on the web.
To the OP/Threadstarter : If you rely on Photoshop for staying in business, I would higly recommend going back to Tiger or earlier. OS Upgrades have sometimes those unpredictible side-effects on both 3rd party hardware and software.