Reply To: Featured images in category view?

Add:

.archive .post header {
  display: block;
  float: right;
  width: 67%;
}
.archive .post .entry-summary {
  clear: both;
}

Update your .entry-summary img style to:

.archive .entry-summary img {
  float: left;
  height: 150px;
  margin-right: 10px;
  margin-top: -61px;
  width: 200px;
}

And finally, add:

@media all and ( max-width: 999px ) {

.archive .post header {
  width: 100%;
}

.archive .entry-summary img {
  float: none;
  margin-top: 0;
}

}