Thanks for your ideas AllanR
"Set color": I already use it.
For now I am using the effect "Outline"
construct.net/en/forum/extending-construct-2/effects-31/effect-outline-53089
But I do not like the result:
There is no problem in duplicating all the images, but I do not think this will improve the result.
then you will need border pieces for each edge of each piece and work out what to show.
Make an image of each edge is the solution, but it's... impossible. As you can see in the screenshot, it would be an exaggerated number of pieces for each map.