For example, you could redesign the Necropolis’s skeleton production building into a Lich production building!
Added after 9 minutes
I noticed that there is no white background in the display on the map. But it is still "blue"... Why?
Attachment 73318
But this won't fix the white background, as you noted - there is no white background on the small image; it disappears when reduced.
I usually remove the background from the image while it is large, reduce it to 256x256, create an outline of 4 pixels, and then reduce it again to 128x128.
Added after 27 minutes
Only two consecutive reductions slightly degrade the quality; I should explain how I do the outlining... But I’m not at my computer