>>2014503setting
for_window [class="^.*"] border pixel 0 will give you this effect in your i3 config (~/.config/i3/config)
it sets the border and titlebar width to 0 pixels
this is explaned on the i3-gaps github page otherwise your i3-gaps will not be presented properly
it could also be due to your image viewer if this is already in your config
if it is i would recomend using feh (probs what the other anons use aswell), no setup required as this clean look is default
