So you are using iPhoto. Sorry, thought you were a Windows user and had to sync with a folder. No go excuse, as your post says you are using an iMac.
Ignore the folder stuff. In iPhoto your choice is to sync the entire iPhoto library, or just selected albums. You set up albums in iPhoto. If you are not sure how to do that use the Help menu in iPhoto.
You select which albums to sync (or the entire library) in iTunes. My link in the previous post should have covered the iTunes setup for both folder and iPhotos.
To be certain I know:
Did you turn off syncing, sync, then turn it on and sync again?
Are you trying to sync the entire iPhoto's library, or just selected albums?
And did you locate and delete the iPod Photo Cache folder as described in the link?
Are you running the current version of iTunes?
And since you are using iPhoto, is it up to date? You don't necessarily have to have the newest version, but that version should have all it's updates. I'm still on iPhoto 9, and it works fine with version 8.1.2
When you look at the info for each file in iPhoto that does, and does not sync, can you see anything common between hem?