I think we may have already talked about this on the discord, but I'll mention it here too.
This is a bug that seems to appear on Chrome specifically, and it's related to a CSS rule that's (I think, god knows it's been a while since I wrote it) supposed to keep the image in a correct aspect ratio. With some specific images at some specific widths, the rounding that the browser does causes there to be a single pixel of gap between the two images.
Annoying as it may be, I don't really have a quick fix for it, and it doesn't really cause any actual issues so it's not a priority to fix. If anyone wants to figure it out for me, please do.