To delete photos from an iPod classic:
Connect your iPod to your Mac and open iTunes (Finder > Applications).
On the left sidebar, click on your iPod to open its menu.
Under Options, unmark Manually manage music and Enable disk use.
Navigate to the Music tab and mark Sync Music. Then, click Selected playlists, artists, and genres.
Unmark the boxes beside the songs or genre you want to delete. Then, click Apply.