Hi there i just had the same headache like yours but i am relieved coz i solved it so fast. Just yesterday i reset my phone and it is set to autoback up files including photos. And as i checked on my gallery, i saw an unwanted photo i tried to delete it many times in many ways but its still remains the same. So here is how i deleted autoback up photos on my gallery (using Samsung S4 mini) :
Go to your "gallery", then tap below screen (left side of the menu) normally use to change for settings, then you can see "content to display" tap on it and uncheck on it from the box then there you go problem SOLVED. Then now you check on your "gallery" and wear a big smile of happiness
Hope this will work for you!
Sincerely yours,
Caren Jopia