Photo Gallery display thumbnails but not full pictures

Hi,

When I try to view pictures using photo gallery it opens up fine into the
screen with thumbnails on. My problems start when I double click on an image
to view it full size. It opens up the full-size window but no image and
sometimes I get a message saying:

This picture can't be displayed. You might not have enough memory available.
Close some programs or, if your hard disk is almost full, free some disk
space and try again.

On other occassions I just get the cursor with the blue 'thinking-about-it'
circle and still no picture is displayed. I have 2gb RAM, virtual memory set
to computer-optimised and 379gb free hard disk space. can anyone help?

Thank you all in advance! David