I first used this app 2 years ago and it seemed to work fine. But then, I experienced the same problems as most other users. I ended up wit LOTS of pictures that only displayed as "!" inside a big outlined grey box when trying to view them full size. The thumbnails are fine, but clicking the thumbnail to see the larger view was a no-go. The good news is, is that the pictures are all still on your computer, in your iPhoto folder. You have to copy the file (picture's) name and paste it into a Spotlight search. Then, when Spotlight brings it up, drag it into iPhoto right where it should be. Then delete the other one (that brings up the "!"). It's a hassle, but it's a fix. Now, I wrote the developer, asking when there would be an update. He responded right away. (I was surprised, because a lot of people were saying he doesn't respond and was slacking. So, here is my email to him, and his response. I hope this helps anyone out there looking for answers.

Me: "Are you updating this app? It seems to really screw up libraries. It's
a great app, but if you're not doing anything with it, please take it
sown at this point. It hasn't worked in years.

Thanks. VocalVirgo"

Him: "Hi Virgo,

Yes, I am working on an update for iPhoto 7. Sorry to hear about your
bad experiences with the current version of iPhoto Diet;
unfortunately, the use of GUI Scripting (a technique that iPhoto Diet
uses to remove unwanted originals) appears to be causing major
problems for some users, while for others it works without a hitch. I
am revisiting the code to see if I can replace GUI Scripting with any
other method; unfortunately, the limited programming interface
provided by iPhoto doesn't seem to present any other options. I will
keep the iPhoto Diet web-page updated as things progress.

All the best, Martin"