Here's something to try. It may seem like a kludge, but it's what Roll20 does on its conversions for other reasons. I was under the impression at first that these were identical tiles, but looking back at the top post, it appears that they are all slices of a larger image? If so, here is what Roll20 does in such situations: In addition to the slices (tiles), create one file of the original full image, at ridiculously low resolution (like 20 px per unit is enough). Place that at full size behind all the tiles. The reason for this practice is: It will typically load before all of the individual slices, cueing those with slower connections that all of the High resolution pieces are on their way (no looking at a blank screen while loading) Being the largest image on the page, it will provide the thumbnail for that page, rather than only one section doing so. Those are the reasons for the practice. As a bonus, it should fill in all of the "gap" areas with pixel data and cause the seams to disappear. Since I follow this practice for the above two reasons, this may explain why you are seeing gaps and I am not.
