CS4 is the most recent version that will run if it's a PowerPC Mac (which presumably it is as 10.6 Snow Leopard is a vastly better OS for Intel Macs).
The older CS2 can be downloaded from Adobe for free -
https://helpx.adobe.com/creative-suite/kb/cs2-product-downloads.html (sign in with a free Adobe account when prompted) - though be aware that as stated at the top of the page it's only technically "legal" if you have original discs for CS2 which can no longer be used because the activation servers are now offline. CS2 is a bit old for 10.5 Leopard but should work ok.