[Logician]

no-hack solution: Create a new style and apply this style to that forum. In that style design your pages in anyway you like.

But if it's only one button you'll change you may regard considering the "hack solution" which is:

put a variable in the template where your button is like $my_button_variable.

Then in the .php file where it calls it, put a line to assign the value to that variable like:

if ($forumid==X) {$my_button_variable='<img scr.........>';}
else {$my_button_variable='<img scr.........>';}

(if ($forumid==X) part may change according to the file you apply this code.)