body { background-color: #0066ff;}
#img1 {  margin: auto; width:60%; margin-top: 20px;}
#img2 {  margin: auto; width:70%; margin-top: 20px;}
.mydiv { text-align:center;   }
@media all and (min-width: 100px) and (max-width: 320px) { .mydiv { margin: auto;  width:100%; } }
@media all and (min-width: 321px) and (max-width: 641px) { .mydiv { margin: auto;  width:75%; }  }
@media all and (min-width: 642px) and (max-width: 1280px) { .mydiv { margin: auto;  width:50%; }  }
@media all and (min-width: 1281px)  { .mydiv { margin: auto;  width:35%; } }