That's an indication that iPhoto has broken the file path/link tetween the thumbnail and original photos. Thereore, make a temporary, backup copy of your library if you don't already have one (Control-click on the library and select Duplicate from the contextual menu) and apply the two fixes below in order as needed:
Fix #1
Launch iPhoto with the Command+Option keys held down and rebuild the library.
Since only one option can be run at a time start with Option #1, followed by #3 and then #4 as needed.
Fix #2
Using iPhoto Library Manager to Rebuild Your iPhoto Library
Download iPhoto Library Manager and launch.
Click on the Add Library button, navigate to your Home/Pictures folder and select your iPhoto Library folder.
Now that the library is listed in the left hand pane of iPLM, click on your library and go to the File ➙ Rebuild Library menu option
In the next window name the new library and select the location you want it to be placed.
Click on the Create button.
Note: This creates a new library based on the LIbraryData.xml file in the library and will recover Events, Albums, keywords, titles and comments. However, books, calendars, cards and slideshows will be lost. The original library will be left untouched for further attempts at fixing the problem or in case the rebuilt library is not satisfactory.
