Is there a method to what pattern is displayed when no profile picture is set? I noticed different patterns for different people.
This is generated by gravatar automatically in the absence of an image, whatever method they use is on that side (as opposed to SO doing it). It works by taking your email (the one you provided to the SO/SE site) and making an md5 hash of it, it then just turns that into a URL, like this:
If it's not on there, you get the generic gravatar icon:
So the method is ultimately your email address being translated into an image based on the hash of it...how exactly they do this it doesn't say.
You can actually change which pattern is displayed by changing the hash in the URL and change the size by varying the number after the
Try fiddling around with different numbers in the hash to see the different colors and patterns that are generated for your liking.
change it to