iPhoto is a great photo management app, which is free for all Mac users. While on one hand, it's pretty capable (in other words, it can handle pretty much any number of photos you can throw at it), on the other hand, it tends to begin having some stability issues with massive libraries. What's "massive"? Well, it's in the eye of the beholder. My library, for example, has 20,000 photos in it (no, that's not all of my photos... I have archived most of them). I've seen some with 100,000 or more.
One of the issues that you may run into, especially after upgrading iPhoto to a newer version, is with the library's database. This database is what stores all the raw photo files, thumbnail images, metadata, etc. Basically everything about your photos. If this gets corrupt, you can run into issues with errors, photos not showing up properly, the application Quitting unexpectedly, etc. But don't stress... there's an easy fix (and a secret menu) that you probably don't know about... until now!
Before opening the iPhoto application, press and hold the Option & Command keys on your keyboard. With both keys being pressed, open the application (be sure to continue holding the keys). As the application opens (but before your library is fully accessible), a new (hidden) menu will appear.
Using these options, you can perform several important first aid functions to help keep your library in good health. The most common function that I see people use if the "Repair Database" utility. As you can see from the description above, this will verify the stability of the database and make any repairs necessary.
In most cases when you have issues with iPhoto, the "Repair Database" utility will resolve the issue. In some extreme cases, you may need to take it a step further and "Rebuild Database". This completely rebuilds your database, overwriting the existing database.
If the only problem you are having with your library is that your thumbnail images aren't showing up properly (not showing recent edits, etc), give the "Rebuild Thumbnails" utility a shot.
Before running any of these utilities, it's always a good idea to backup your iPhoto library first!! If you're not using Time Machine to backup your system, PLEASE DO!!!