Images getting squashed

I am sorry if this is a basic question but I want to display my images in my blog posts at 800 x 533. I add the images and the thumbnail appears. If you click on the thumbnail the image is not displayed in full . In the past it has been and I am confused as to what i have changed to stop the image displaying as I want. There is an example http://hampshirephotographers.co.uk/2011/08/28/joseph-family-portrait/0072_joseph/

HELP please