Macintosh was first to make a big splash with a GUI, so naturally this started the perception that they were better for "graphical jobs."
If you've ever worked in the print industry, and compared the problems trying to get documents created on the PC to print properly compared to those created on Macs, you'd see why this perception continued even after Windows 3.1 became popular.
Historically, Macs had better font handling, colour workflow, postscript support, anti-aliasing, and other little technical details which made video and graphics creation smoother on the Macintosh platform.
I haven't heard this one for a while actually, which is probably partially due to Mac now using Intel Processors.
The switch to Intel is fairly irrelevant. You probably haven't heard this in a while because later versions of Windows solved many shortcomings and the proliferation of discrete graphic GPUs made Windows PC boxes graphical powerhouses, in the raw pixel-pushing sense.