when syncing with my ipod touch, why does itunes say "optimizing photos" even if nothing has changed .
When you sync photos to an iPod or iOS device iTunes performs a set number of tasks, which include optimizing photos, if photos are selected to sync and to be optimized. iTunes doesn't cache the optimized photos, and if anything on the device has changed (like a contact, or some application data, basically anything) then it will sync all of the photos again, including optimizing them. 

Basically, it will do this every single time you sync, and there really isn't much you can do to stop it. You could try to restore your device and NOT restore from backup, that might work, but is it really worth it?