.main{
	//background-image: url(http://www.forum-ims.org/portail/images/portail.png);
	//background-repeat: no-repeat;
	position: absolute;
	width: 550px;        /* selon la largeur voulue */
	margin-left: -275px; /* moitie de width */
	left: 50%;           /* constant, toujours 50% */
	height: 550px;       /* selon la quantite de texte */
	margin-top: -275px;   /* moitie de height */
	top: 50%; 
}

.link{
	width: 550px;
	height: 550px;  
}

.forum-link{
	position: absolute;
	bottom: 0px;
	width: 70px;
	height: 70px;
	margin-left: 360px;
	margin-bottom: 70px;
}

.forum-link img:hover{
	background-image: url(http://www.forum-ims.org/portail/images/forum-logo-background.png);
}

.explanation{
	display:none;
	position: absolute;
	bottom: 0px;
	border: 1px solid #000;
	height:120px;
	width:350px;
	margin-bottom: 320px;
	margin-left:-240px;
}

.explanation .link img:hover{
	background-color: #F0F;
}