This is happening in the simplest, smallest version (Inger's project) and therefore I figured that's the best and easiest place to investigate it.
All I did to Inger's project was change the border of the main pics from double to solid and from 3px to 2px. And then on the modal I changed a bunch of stuff - but that shouldn't be affecting the main pictures.
Here is another screen shot with Inger's original pics (and none of my pics):
The gap is on every one of these pics. And if I drag my browser window smaller, the gaps move and appear on sides of the pics they didn't appear on before.