Most probably the image was generated dynamically using PHP or something similar, which can obtain information about you from your browser (user agent), like phpSniff
Mine is mozilla/5.0 (windows; u; windows nt 5.1; en-us; rv:1.7.12) gecko/20050915 firefox/1.0.7
which shows I'm running Firefox 1.0.7 on Windows XP (NT 5.1).
There is an extension for FF which allows you to fake your user agent.
As for the ISP - your ISP is allocated a specific block of IP addresses to give to users. The information (which IP blocks belong to which ISP) is available freely on the web.
A good example of good use of this data is BBClone
- an advanced web counter.