Personally I think that closing a single image by clicking on it it's a good idea since it will minimize mouse movements (and improve ux on small touch screens where it's hard sometimes to tap the close button).
Concerning multiple images I think that clicking an image should open next image because of the same reason (and close or restart the gallery at the end).
It's always a good idea to let users do something with minimum movement.
BTW, I think that it's better to place close button at the corner of the image because placing it at the corner of the screen will require more movement (many notnot experienced users will try to click the cross instead of clicking somewhere else to close an image).