An image someone sent me to my gmail, has ended up on my phone gallery, I am unable to delete. Any ideas?
It is possible that due to it being in your Gmail, and your account being set up on the phone, that when you try to delete it, Gmail resyncs it to your device.
So my advice would be to log into Gmail.com and remove the email from there, making sure you remove it from trash as well. Then attempt to remove the picture from your gallery.