Try these styles out.
#wmd-preview img:not(.thumbnail), .cooked img:not(.thumbnail) {
max-width: 100%; /*This needs to change from 690px to 100%;*/
max-height: 500px;
}
@media screen and (max-width: 960px) {
.topic-body {
width: 100%;/*this needs to change from 750px to 100%;*/
}
}
The .topic-body is the main culprit. But Mitts screenshot was showing very very slight scrollbars so it needs a revamp on that image rule as well.
Might want to put that image rule inside of the media query as well (the .topic-body media query above). Less will be affected.