It is a bad idea to try to manually modify the folder structure inside the iPhoto library, as you seem to want to do.
The best way is to follow Apple's instructions for rebuilding the iPhoto library.
However, this is usually only used as a repair procedure if iPhoto is malfunctioning.
You can try this, but make sure you have a complete backup of your iPhoto library before you start the rebuilding procedure.
Also, realize that iPhoto has an internal Trash that is separate from Mac OS X's Trash. If you have deleted hundreds of photos over the years, and never emptied the iPhoto Trash, those photos will still be in there and taking up space in the file structure.
Most painless way: Restore from back up.
Assuming the photos haven't actually been deleted, but instead some form of database corruption:
Option 1
Back Up and try rebuild the library: hold down the command and option (or alt) keys while launching iPhoto. Use the resulting dialogue to rebuild. Choose to Repair Database. If that doesn't help, then try again, this time using Rebuild Database.
If that fails:
Option 2
Download iPhoto Library Manager and use its rebuild function. (In Library Manager it's the FIle -> Rebuild command)
This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.
Regards
TD
Best Answer
By design intent, files stored in
~/Library/Cachesshould always be safe to delete, at least as long as the application isn't running. You can mitigate any poor programming bugs by moving them to trash and then rebooting the mac before you empty the trash as this ensures whatever programs are running can still write files to that old cache directory but will be forced to make a new Cache folder the next time it starts.In your specific instance, the folder/files in question are created by iPhoto when you start a slide show. It seems that they do not get removed when you exit iPhoto later on so you have to remove them manually if you want to recover the disk space.