It's not about the photo filters, not anymore. Here's a rundown of why Macs have this stereotype about being used by designers

First, the obsolete 'facts'
a million years ago it was the only operating system with a real GUI, and the only one that supported a mouse.
Photoshop has always been available (and for a long time better supported) on Macs

And now, onto modern day
They have this stereotype associated with them already due to the previous two points, and these things tend to perpetuate themselves. Design shops buy Macs because they have always bought Macs
Designers have no need for the office suites or enterprise integration that are Microsoft's strong points
Designers care about aesthetics, and Macs have the most aesthetically pleasing hardware and software available
Designers typically don't know anything about computers other than Photoshop, they want Photoshop to work with the least amount of stress and Macs are great at 'just working'
Apple makes the best displays in the industry and they have for years
Apple bundles a bunch of multimedia stuff into the OS, but I doubt any of this is of use to a real designer
However, they also offer higher end applications like Aperture which are only available for Mac

I hope this clears things up because frankly I'm sick of all the misinformation flying around itt