@media only screen and (max-width: 500px) 
{
.bg
{
width:120px; 
height:300px;
float:left;
background:#FF0000;
margin-top:250px;
margin-left:300px;

}
}
@media only screen and (min-width: 510px) 
{

}
@media only screen and (min-width: 768px) 
{

}
@media only screen and (min-width: 1200px) 
{

}