Bouton qui reste allumé

<a href="javascript:"><img id="b1" src="images/boutons/presentation.png" style="border:none" onMouseOver="Over(0,this.id)" onMouseOut="Out(this.id)"/></a>  

var mes_images = new Array();  

var liste = new Array("images/boutons/presentation2");  

var longueur = liste.length - 1;  

for(im=0;im<=longueur;im++){  

mes_images[im] = new Image();  
mes_images[im].src = liste[im];  
}  

image_temp = new Image();  

function Over(indice,id_img){  

image_temp.src = document.getElementById(id_img).src  

document.getElementById(id_img).src = mes_images[indice].src;  
}  

function Out(id_img){  

document.getElementById(id_img).src = image_temp.src;  
}