LOL I was actually messing around with that today, It's actually very easy to do (even if you want to use your own font)
If you look at mine, (from pixelpets.co.cc :D) I set mine to use a font i downloaded for my recent obsession (Simlish from The Sims game series)
1. Find a font (I've had luck with Google and dafont.com)
2. You will most likely have to convert the font to a .gdf file so I reccomend http://www.wedwick.com/wftopf.exe if its a TrueType font.
3. Upload the font to the fonts folder on your server.
4. Open up siggy.php and look for the following lines (around Line 170)
PHP Code:
$str1 = "Name: ".$name;
$str2 = "Owner: ".$owner;
$str3 = "Train Me!";
$str4 = "More Adopts at:";
$str5 = $domain;
imagestring ($image, 12, 0, $textheight, $str1, $color);
imagestring ($image, 12, 0, $textheight + 13, $str2, $color);
imagestring ($image, 12, 0, $textheight + 26, $str3, $color);
Yours is probably different because I have edited mine a bit
5. Insert the following code (with obvious modifications BEFORE the imagestring() lines
PHP Code:
$font = imageloadfont('fonts/GDF FILE NAME HERE!!!!!.gdf');
That tells the page to load the font for usage
6. Change the 2nd variable in the imagestring lines you want this font to be used (in this case replace the 12s)
In the end it should look somewhat similar to this
PHP Code:
$str1 = "Name: ".$name;
$str2 = "Owner: ".$owner;
$str3 = "Train Me!";
$str4 = "More Adopts at:";
$str5 = $domain;
$str6 = "LOL Simlish";
$font = imageloadfont('fonts/sims.gdf');
imagestring ($image, $font, 0, $textheight, $str1, $color);
imagestring ($image, $font, 0, $textheight + 13, $str2, $color);
imagestring ($image, $font, 0, $textheight + 26, $str3, $color);
imagestring ($image, 12, 0, $textheight + 43, $str6, $color);
Sorry that this tutorial is kinda crappy im a bit tired at the moment