create a link in footer "*** to top" and add url with value - #

another method: use google search "scroll to top jquery" and add html code 

Fabrice,

Use any picture you want, set it in the below part of your site or footer as Aleksey suggest).

Then add a link to it. (you can make a pop up descriptin as well in the link page use TAB Description in link menu)

Then link it to the page your on (website page).