I know that imgur.com provides the service to SO... But from the imgur API, the return from the POST is a JSON, so I guess the API URL (http://api.imgur.com/...) can't be written to the ACTION field of a FORM — otherwise after the upload the browser will receive a JSON document.
SO posts to /image/upload, I can't understand how the image data is proxied to imgur.com, that must be very interesting!
Fiddler shows the POST:
POST http://stackoverflow.com/upload/image HTTP/1.0
Host: stackoverflow.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; zh-CN; rv:1.9.2.12) Gecko/20101026 Firefox/3.6.12
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: zh-cn,zh;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: GB2312,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Referer: http://stackoverflow.com/questions/ask
Cookie: __qca=P0-1495010330-1282271917039; __utma=140029553.1957082693.1282271917.1290407105.1290413891.456; __utmz=140029553.1290392269.453.134.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=actionscript%20how%20to%20generate%20uuid; m=5; usr=t=r4bTciiabke0&s=g7Av0fdob0ql; __utmc=140029553; __utmb=140029553.3.10.1290413891
Content-Type: multipart/form-data; boundary=---------------------------276212999315826
Content-Length: 469897