Can an unknown picture show up on my smartphone?

with photos> Gallery pics are all mine but there is an unknown pic in photos. Out of 4 pics 1 is not mine.

What do you mean?

if you bought the phone 2nd hand and the previous owner forgot to factory reset the phone then it's possible you have stumbled across one of their pictures, but as neonworm has stated above, can you elaborate a tad?

Gallery shows any picture format files (bmp, jpg, png, etc.) that are on the phone - including the ones that apps and browsers download for their own use if they're not written correctly. (Any app downloading pictures for its own use should add a .nomedia file to the folder it's downloading to - a sloppy developer may have forgotten to do that, leaving the pictures visible.)