In Messages 7.0.1 on the Mac, is it possible to delete specific messages in a chat session?
I found out how to do it: single click on the message/picture you want to delete so that it is highlighted. Then, right-click and you will be given the option to "Forward...", "Copy", or "Delete..." the message. Click "Delete...".

Note: Doing so will ONLY delete that message from Messages on your Mac. If you want to delete the same message on any particular iOS device (iPhone, iPod Touch, iPad), you have to delete it on each device individually.