'I have been having trouble with I photo' - the application is 'iPhoto'.
'I cannot see any pictures on the largest size which is not very useful when I am trying to review them. I get a grey box with an exclamation mark.' - this is an indication that the original file(s) has / hav been moved, from where 'iPhoto' has placed it.
Consult '
iPhoto 6: Image appears when browsing but is blank or exclamation mark when editing' for instructions to resolve your problem.