The "display:moz-box;" applied to .post-holder is preventing the images from showing up. Looking into WHY now.
I haven't messed with flex-box layouts much at all, so this is largely assumptions. You have a "display:mox-box;" set on .post-holder, but .post-image neglects to set any flexbox properties. Adding "-moz-box-flex: 1;" to .post-image causes the images to display, but likely are not the size you're shooting for.