Why do my photos appear as an exclamation mark when i open them?

Question

Why do my photos appear as an exclamation mark when I open them?
I used my time machine back up from my old expired power mac and uploaded the contents onto my new iMac.
All of the files uploaded successfully as far as I can see (except the old programs don't work and the consequently all those files are cactus :-( )however when I opened iPhoto only a few photos (500) where in some of the folders.
I went back and found the photos in the back up machine (25000) and dumped them on the iPhoto icon. They down loaded successfully into the correct folders and the thumbprints are visible. When I click on an individual photo the enlarged picture appears for an instant and then an explanation mark in a triangle appears in its place. 
The original 500 pics actual worked... now after days of mucking around and with lots of advice I have doubled the number of 'photos' to 50000!!! Ahh,,,, and still the exclamation mark appears
Is there anyway out of this predicament??

Answer 1

This went wrong here:
I went back and found the photos in the back up machine (25000) and dumped them on the iPhoto icon.
Never import one Library to another. Every version and thumbnail is imported like a distinct photo, you lose all your Albums, Keywords etc., the link between Original and Previews is destroyed, the non-destructive editing feature is ruined and so on. In summary: it's mess.
The ! turns up when iPhoto loses the link to the original photo.
I would suggest that you restore that library from Time Machine again, and then try repair it.
Download iPhoto Library Manager and use its rebuild function. (In early versions of Library Manager it's the File -> Rebuild command. In later versions it's under the Library menu.)
This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.