Do you just generate random patterns and give them to users, which is somehow non repeating? Also, where can I find more about this. Thanks.
That is called a Gravatar, which by default is generated by hashing your e-mail address. To change it you go to your account page (by clicking on your own username), click on "edit", and there is a link available for you to change your picture.
Theoretically, as a hash, the Gravatar is not unique. But the probability of actual collision is really, really low.
$\begingroup$ No, I didnt make it clear, then. My qyestion was, is this a non-repeating pattern? Is it unique for all users (have no interest in changing it or anything) $\endgroup$ Apr 23, 2011 at 16:57
4$\begingroup$ @yayu: Willie explained this in his answer, I will leave the proof of that as an exercise to the reader... $\endgroup$– Asaf Karagila ModApr 23, 2011 at 16:59
$\begingroup$ I see. By what process can someone automatically generate so many unique pictures, with low possibility of duplication? $\endgroup$ Apr 23, 2011 at 16:59
1$\begingroup$ @Willie One needs to distinguish between mathematical collision vs. end-result visual collision - which depends on many non-mathematical factors (screen resolution, visual acuity, etc). E.g. on screens that are smaller or lower resolution (e.g. phones) there's a higher probability of visual collision. $\endgroup$ Apr 23, 2011 at 18:02
$\begingroup$ see en.wikipedia.org/wiki/Identicon $\endgroup$ Apr 26, 2011 at 9:47
$\begingroup$ As suggested in Jeff's comment, the automatically-generated Gravatars are actually a specific subfeature of Gravatars called "Identicons." $\endgroup$– IsaacJun 25, 2011 at 14:21
Googling Gravatar Algorithm from Willie's answer gave me the hint. Here are some other quesstions:
what is the algorithm used to generate those little gravatar identicon images?