I have done research on projects similar to this (like sIFR but using PHP) and I found one that looked quite interesting. You were able to style the image by just using CSS.. very interesting indeed. I had a peek at the source and made the following discoveries...
1. The CSS did not
really style the image.. the PHP was simply setup in order to parse the CSS. To avoid having to open and read the CSS file for every header image the information was stored in sessions (which now requires cookies)
2. Every page required the usage of PHP.. which of course is a hastle for anyone using a static site.. and the PHP that had to occur on each page had to be inserted at the top and bottom... even if you use includes, it is still a bit tedious. The PHP put on the page added the appropriate class attribute and on an advanced version it inserted spans (allowing for image wrapping). This is slow.
3. Styles could not be generated on the fly. You only have five header elements to work with and they absolutely must be the same over your whole site (as styling is stored in sessions).. however, what I have invented allows for any amount of different styling on any amount of different pages. Also, the capability with the CSS (unless using the version written by the other person) was limited as well.
The main discovery I made is that their system was simply a wrapper around their image file (like I have above.. but twice as long with LESS features

). I could potentially write a more efficient wrapper only using ONE php file (one separate from the image) and some rewriting work... however, this introduces (potentially) undesired frameworking.. and could be difficult to integrate. In the end I decided it just was not worth it... a whole lot of extra mess just in order to specify styles in CSS instead of in a URL. It may *look* nicer on the client end but it is a mess for the server end of things. If anyone wanted to write a wrapper script to automate the image file URLs then so be it.. it just is not worth restricting people to it.