Q: need to delete photos that were synched from my pc to ipad. they're taking up too much room.

Those photos can only be deleted from your iPad by connecting your iPad to your iMac, unselecting the photos in iTunes on your iMac, and syncing.

If the photos were added via iTunes sync then the need to be removed by un-syncing them with iTunes.

iTunes: Syncing photos

1. 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.)
2. Click the Photos tab in the resulting window.
3. 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."
4. Click Apply.

You will have to resync using the Mac...photos cannot be deleted when placed on the iOS device by a computer sync.

You have to select your device first: