I'm trying not to get too confused here, so bear with me. Let me make sure I have this right:
1. On your phone, you have Picasa sync checked on in Settings>Accounts-Google.
2. In your Google+ Photos webpage on your computer, you see the photos that you don't want.
3. In your phone's Gallery app, you see those same photos, but can't delete them from there.
Do I have that right? If yes, then on your Google+ Photos webpage on your computer, select the photo you don't want and delete it. That photo was presumably still syncing with your phone's Gallery through Picasa, which is why you still saw it there. Make sure the photos are gone from Google+ Photos--refresh the webpage a couple of times to make sure they don't pop up again for some reason.
After deleting the photo on your Google+ Photos webpage, now go through that same process we went through before of Clearing Cache/Data from the phone's Gallery app, then Force Stopping. Open Gallery again, and see if the unwanted photos show up again.