This is simpler then you think
He did not mean to modify a file.
He meant:
Goto the admin control panel,
browse from the left menu down to the 'replacement' section
and click on 'modify' to find the {imagesfolder} replacement
Then, if you see it there and it says forum/images (or dir/imagesdir) then you dont have to modify the file
but if it says
http://www.yourforumsite.com/forum/images/) (a full URL) then you have to modify the file.
in 99% you dont have to modify anything