Outlook 2013 – No longer able to view pictures in emails

Hi, I have been using Outlook 2013 for a few months without any problems. All of a sudden my pictures stopped displaying in my emails - both incoming and outgoing. There is no red 'x' in the corner - I just see a plain box outline where the picture should be except I cannot view these pictures. When I forward the same email to another email address I can view the pictures okay. HELP please - this is driving me mad trying to work out what the problem is.