Thanks Dandello
I have rechecked the CSS for both lytebox and thumbs and found two errors an uneccessary comma and using font-color instead of just color! I am getting a css validatation error "background too many values or values not recognized . . ." and can't find any reason for those errors.
I don't think that this is a border issue as the first image has a border-bottom set to 10px and red but still shows the grey bottom border, which I suppose is not possible as there cannot be two bottom borders can there? So it would seem that the grey bottom border is a margin.
However, I agree there appears to be a faint border around the thumbnail images only.
Geoff