If you've selected a thumbnail and used the File ➙ Show in Finder ➙ Original menu option and iPhoto didn't find the orignal then it's gone.
If the library was in the reference mode for those photos and them came directly from a camera they are gone for good unless they are still on the camera.
If you get this
when you double click on one of the thumbnails the following may restore them to the library: 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.
If you want to determine if the orignal file is somewhere on the hard drive get them file name from the thumbnail and do the following:
1 - download and launch Find Any File
2 - launch FAF and search for the file name. FAF can search areas that Spotlight can't, like invisible folders and packages.
3 - if you find the originals somewhere on the HD make sure iPhoto is set for a "managed" library and import the new found photos into the library.
OT