function change_img()
{
dark.className='pic'+(Math.floor(Math.random()*4));
}
setInterval("change_img()",10000);
<!--
if (window.Event)  
document.captureEvents(Event.MOUSEUP);  

function nocontextmenu()  
{ 
event.cancelBubble = true 
event.returnValue = false; 

return false; 
} 

function norightclick(e)  
{ 
if (window.Event)  
{ 
if (e.which == 2 || e.which == 3) 
return false; 
} 
else 
if (event.button == 2 || event.button == 3) 
{ 
event.cancelBubble = true 
event.returnValue = false; 
return false; 
} 

} 

document.oncontextmenu = nocontextmenu; // for IE5+ 
document.onmousedown = norightclick; // for all others 
//-->
function showUsermenu(i)
{for(j=1;j<=3;j++)
{
eval("document.getElementById(\"Usermenu"+j+"\").style.display=\"none\";");
}
eval("document.getElementById(\"Usermenu"+i+"\").style.display=\"\";");
}

function showAcmenu(i)
{for(j=1;j<=3;j++)
{
eval("document.getElementById(\"Acmenu"+j+"\").style.display=\"none\";");
}
eval("document.getElementById(\"Acmenu"+i+"\").style.display=\"\";");
}

function showComicmenu(i)
{for(j=1;j<=2;j++)
{
eval("document.getElementById(\"Comicmenu"+j+"\").style.display=\"none\";");
}
eval("document.getElementById(\"Comicmenu"+i+"\").style.display=\"\";");
}