I know this post is old but it's the first one that came up in Google search when I was desparately searching for a way to find my lost photos. This post has helped me to find all my missing photos so I feel I should share what I did in case it might be useful for some people.
Basically I had the same exact scenario with the original poster. I was importing about 5000 photos from my iPhone using iPhoto. Got the pop up message saying that all photos were successfully imported and asked if I want to delete the photos from my phone. I clicked "Delete" (big mistake!). When I checked my computer 10 minutes later, I saw that iPhoto was crashed and quitted unexpectedly. Reopened iPhotos, none of my imported photos were there. Checked the phone and there were only 900 photos left. I've lost over 4000 photos somewhere.
I tried search for the images on the computer and got nothing. Then I googled and found this post. First, I tried rebuild iPhoto database. That didn't work. I also couldn't see the "Previews" or "Masters" folders as mentioned early in this post. When I clicked on iPhoto, it just opened up the iPhoto applications. There was no folder there.
Per leonie suggestion, I installed "Find any File" and searched for "Library6.iPhoto". The app found it but it didn't do anything. When I tried to go there it just opened up the iPhoto. So I tried the following:
I know that the file name would be something like "IMG_xxxx.jpg". So I used Find any File app to search for "IMG" and set the date to be within 2 years (in case it uses the date that the photo was taken) and I found them all! I could see all my missing images through this app even though I couldn't find them by using the search bar on the finder. And the weird thing is, when I clicked on the photo (still on the app), the app would show the location of the photos. They were in the folder "Originals" under the iPhoto Library. However, if I go to iPhoto Library directly on the computer, the folder is not there. It's like it's invisible (search result for "Originals" also came up zero) and I can only see it through the app. But I was able to double click the "Originals" folder on the app and it opens up a window that contains all of my images.
I don't know why it has to be this complicated just to locate my files but I am very grateful for this post that helps me find all of my missing images.