To delete synced photos and videos from your device
In iTunes, click the Device button in the upper right corner. (If viewing the iTunes Store, click the Library button in the upper-right corner first.)
Click the Photos tab in the resulting window.
Choose "Selected albums" and deselect the albums or collections you want to delete.
Note: To delete all synced photos, deselect "Sync Photos from" and, when asked, click "Remove photos."
Click Apply.
(Apple Support)