When you go to Google on your PC, at the top they have their shortcut bar. Click on More, and then probably need to go to Even More at the bottom. Should have Picasa in the list. Assuming you're logged into your Google account on your PC, it should open up and let you delete your pictures there, if they're still there. Also, check your Google+ account and see if they're there instead. Delete them there, and then give your phone a bit to sync, and they should go away. If they aren't there at all, in either place online, then the only other option would be a file manager as suggested, to delete any copy stored on your phone.