Windows - should I make WWW pictures a bit dark or light to accommodate Mac users?

Don’t worry about it. But if you care enabling sRBG when you save the image should take care of things (or at least them make them irrelevant).

Too many variables to worry about it.

Keeping your monitor calibrated to standard Gamma, and creating the images just the way you want them to look, should be good enough I guess.

If I remember correctly, IE6 and below have issues rendering PNGs (they render a bit darker) – so avoid this browser.

Most Macs now use a more standard Gamma setting so the difference is not as dramatic as it used to be.

Yeah, what @sharpstick said.

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tips4mac [dot] com noted:

The default setting in (Mac OS X) 10.5 is 1.8 and it is 2.2 in 10.6 for the color gamma. The new default is the same than what Windows has been using for years. What this means is that websites will finally have the same colors on a Mac and a PC (much easier for web design).

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So any explicit optimization you do is for older Macs, not up-to-date ones.

This Panic blog post is particularly elucidating about optimizing graphics for Windows and Mac OS X 10.6 gamma.