This is almost a reprise of my earlier posting on the last version of Photoshop supported for the last PowerPC Mac. In this case, it is the same topic, but for 68K Macs.
The quick answer is Photoshop 4.0. It took a bit of digging to be sure, since some sources erroneously reported that Photoshop 5.0 would also run on 68K Macs, but Adobe’s own “System Requirements” page set me straight. This version of Photoshop is now executing on both my Quadra 660AV and my Quadra 840AV, and so I can report with confidence that it does work very well indeed on 68K Macs.
So, for the record, the highest version of Photoshop that you can run on a 68K Mac is Photoshop 4.0.
Nice to know, eh? 🙂