I want to know how Doctype's avator images have rounded corners. I saw a strange url in firebug so I copied and pasted the url into the address bar and hit enter, then I saw that the image appears without round corners so it's not rebuilt on the server. I already can do this with php but the result image isn't sharp. I hope I can learn this technique :) Sorry about my bad english :)
They're doing it with CSS 3:
It only works in Firefox and WebKit-based browsers like Safari and Chrome, though. You can read more about border-radius and other CSS 3 features here.
If you don't want to use JS or CSS3 you can always use the old technique of Mountain Top Corners. The article linked to uses 1 bit alpha transparency with a GIF but you could use a PNG nowadays (maybe swapping the PNG for a GIF in IE6 if you want to be nice). All of the mentioned techniques have pros and cons so a combination of them may be the most robust solution.
you could get clever and have the avatar as a background on a transparent png image frame.
Or absolutely position a transparent png image (with the middle bit empty/transparent) over the top of the image that needs the rounded border.