I've seen a few threads on this, but unfortunately nothing seems to be working.
We have a custom 2x2 map, and I uploaded all the map files are directed into the ShooterGrid\ServerGrid directory. I noticed with the initial release of the server grid editor that it was exporting PNG files for the cell images, so I tried the trick of just renaming them with JPG extensions, but no luck. What I did find is that when I cleared out my local Saved\MapImagesCache file it downloaded CellImg_0-0, bu