Hey Bryan,
All you need to do is replace the original images that threads uses. Open up a thumbnail view of your images dir on your HDD. Look for the .gif file you want to chage (newpost.gif) etc. Choose the button you would like to use, rename it to newpost.gif and upload it to your server.
In the languages/english dir edit generic.php
Should be somewhere near the begining of the file
$ubbt_lang['NEW_POST_ICON'] = "Post";
put two slashes to comment out the Post
//$ubbt_lang['NEW_POST_ICON'] = "Post";
This stops the word post appearing next to your image.
Thats it.
Do the same for edit, reply or anything else you want to change.
No major script hacking needed.
cheers []/forum/images/icons/cool.gif[/]