Tiled Map Optimizer
Have you created a map with Tiled? (https://www.mapeditor.org)
Does your map include multiple TileSets and you are using just a few tiles from each image?
Then you can use this optimizer to get a single merged PNG image and all the data-layers fixed to use the optimized image.
Got the result image? You can even try to optimize more with: https://tinypng.com