Hi Nihas,
Firstly, welcome to ForumUp English Language Support
Normally, this butons change with HTML code or css. files
But it is different. URL links given under your forum template's banner.
You try this method please;
Go to Administration Panel
Styles Admin
ForumUp Theme
Logo From: templates/lpch_abstract/images/banniere_lpch_abstract.gif
Logo To : http://img176.imageshack.us/img176/5593/bannierelpchabstractls6.png
and finally "change logo" buton.