I've created a simple GIMP file to serve as a template for use existing grid map images, as table images for the Kraken.

There are two layers, one is the actual reference grid, and the other, is a sample layer to show how you can use any 3rd party image on the reference. After importing the external map image, you have to move and scale the image to perfectly fit the reference grid. Then, you can export the image and use it as the diffuse/image of the Kraken background. Grid size of TTS should be set to 1 in both dimensions (it's 2 by default).

Sample map I've used, was generated by donjon tools.

{LINK REMOVED}https://bit.ly/3iH4ObA