For those that would like an answer for this, I got some outstanding assistance, (via another forum BTW.)
[ QUOTE ] the .htaccess is the file that IIS won't like. All you have to do is change sig.jpg to sig.php and link to it in forums by:
CODE
[CENTER][URL="HTTP://WebsiteAddress.EXT/Directorie(s)/FileName.EXT"][IMG]HTTP://WebsiteAddress.EXT/Directorie(s)//Sig.PHP[/IMG][/URL][/CENTER]
If forums won't allow that as an image, you'll have to change what your server thinks of .jpg files. Set it to parse them as php.
To do this in IIS you need to (as far as I know):
open IIS
right click on the folder containing your sig.jpg file
select 'properties'
click on the 'http headers' tab
click 'file types' in the 'MIME map' section
add a new type with
Associated extension: .jpg
Content type (MIME): application/x-httpd-php [ /QUOTE ]