I like the solution you posted, it's straightforward.
Another option to solve this is to open up the tile-map image in an image editor (Photoshop, Gimp, etc) and create a stand-alone sprite image. Save it separately, discard any edits to the original tile-map image, and import the new sprite image into your project.
If you're obsessive about having the smallest file sizes for your images, smaller images like this will help.