Q: pictures won't delete

The only way to get picture off the phone that you have synced from the computer is the same method, only in reverse. You connect the iPhone to the computer and iTunes, select it in iTunes, go to the Photos tab, then find what you have synced. It should have a checkmark by it. Remove the checkmark and sync the device again. See this support document for additional instructions if needed.  Keep your photos safely stored and up to date on all your devices - Apple Support

You need to sync to iTunes again in order to delete. See the instructions below from Keep your photos safely stored and up to date on all your devices - Apple Support

If you don't want the synced photos on your iPhone, iPad, or iPod touch anymore, turn off syncing in iTunes and they'll be removed from your device. You can turn off syncing for some of your photo albums or all of them at once.

Stop syncing some albums

1. Open iTunes on your computer and connect your iPhone, iPad, or iPod touch.
2. Click on the device icon in iTunes.
3. Click Photos.
4. Choose "Selected albums" and deselect the albums or collections that you want to stop syncing.
5. Click Apply.

Stop syncing all photos

1. Open iTunes on your computer and connect your iPhone, iPad, or iPod touch.
2. Click on the device icon in iTunes.
3. Click Photos.
4. Deselect "Sync Photos" then click "Remove photos."
5. Click Apply.

 

Learn how to delete photos and videos synced from iTunes when you can't access the originals.