Two ways to do that :

1, Open: /themes/MSTrenches/header.html

line 5 calls for The index.php is a file used to generate a random image from your backrounds folder, you can either rename that index.php - eg: to one of the images you do like ie: background1.jpg

2, Just delete the backround images you dont want from themes/MSTrenches/forums/images/backgrounds Folder

...or replace 1 or all them with your own