1) You set image width 500px http://prntscr.com/nuu6sb Container width is 1170px. So, 500×2 + 6×4(image paddings with border) + 15×4(columns paddings)= 1084 images are center aligned and you get the gap. Change the images width.
2) Also, you may try Media grid element if you want to add few images http://prntscr.com/nuu5kc It allows you to choose the gap, number of images per row and also image design http://prntscr.com/nuu6cr
