A MacRumors forum poster, "Ardoptres," reports having image management issues:
"I have soooooooooo many duplicate photos on my computer, but I don't know what happens if I delete some of them. I did this to check what would happen.A couple things are happening here. The first thing to understand is where your Mac stores your images. The easiest solution to photo management is iPhoto. Simply connect your supported digital camera and use iPhoto to import your pictures, similar to importing music from compact discs in iTunes. Where many people get confused is what iPhoto does with those image files once they are imported. In Finder, you are probably familiar with the Pictures folder that graces the sidebar.
I open iPhoto, choose a picture and open it (to check if i can open it)
I find all the duplicates of that photo in finder, and delete them.
I try to open the same picture in iPhoto, but now it just shows a blown-up exclamation point.
I go to the the trash can, put all the photos back
Try to open the picture in iPhoto (expecting it to open) but it STILL shows the exclamation point!
I find this to be very weird. The files I deleted are back and yet it still won't recognize it as being there. I have Aperture and I can open the picture there with no problems.
The Pictures folder is simply a preset folder in Finder that is set up to help users organize their files. You will notice the iPhoto Library file resides there. That file can be thought of, simply, as a locked folder. Inside that folder is everything iPhoto needs to operate, including all your pictures, information about albums you create, and previews of any products (like calendars, books, or slideshows) you create. The file is locked because making changes to the structure can severely affect iPhoto's performance.
Where Ardoptres got confused is that they had photos, perhaps from e-mails, downloads from the Internet, or obtained via disc media that resided on their hard drive and in iPhoto. The first thing to remember is that if you import a picture into iPhoto, it will be copied to the iPhoto Library file in your Pictures folder (unless you change the default setting). In this case, Ardoptres can simply delete the other iterations of their pictures from their hard drive.
The other issue at hand was the exclamation point problem in iPhoto. This means that the files related to the thumbnail were not located or linked to properly. To help correct this issue, hold the (Option + Command) keys when launching iPhoto. The following screen will appear:
If your Mac is slower, choose each item separately. Keep in mind that some options may take quite a bit of time depending on the size of your iPhoto Library and the speed of your Mac.