Not really a simple solution for this one. The text comes from the language file, so it's generic to all forums. You'd need to modify the postlist.inc.php script to do this. There is a conditional in the script already where it checks if it's a gallery forum, so it can change the text to New Image. So, you'd need to add a bit of code that checks the forum id and if it matches that one particular forum, then you could change the text.
