I altered the buton code like this.
Make sure to set the location of the img and set the width and height for your button, i used the blue variant.
<div style="
bottom:0;
display:block;
height:65px;
left:0;
position:fixed;
text-align:center;
width:100%;
z-index:999;
cursor:hand;"><img src="http://www.YOURSITE.COM/webim/b.php?i=mblue&lang=nl" border="0" width="177" height="61" alt=""/></div></a><!-- / webim button -->