Best answer as chosen by user tdewolfe: Although you did not mention it,
by mrmacfixit - 10/17/11 4:40 PM
In Reply to: Anyone have troubles viewing iphoto photos after reformat?? by tdewolfe
you probably get some form of warning when you try to "view" a picture by double clicking on it.
The normal cause of this problem is that the picture was not actually in the iPhoto Library when you moved it to the external.
It was living somewhere else on your hard drive and was destroyed in the format.
However, iPhoto had a thumbnail of the original picture, which is what you see before you double click it, but iPhoto cannot find the original, full size, picture.
I hesitate to say this but, why not just reload the unviewable pictures from your Time Machine backup?
As an aside, what made you re-format the internal HD?