I’m not sure what you are trying to do, but I will break down how the overmap works.
Coded into the game is a .png file which contains every ascii symbol used in the overmap. This file is pink and white. The game ignores the pink color by design, the pulls every white pixel in a symbol, an 8x8 square, or whatever size it is, then places them on the over map.
After generating, the game looks up what is in an overmap tile. It assigns a symbol and a color, as denoted by the config file found inside the relevant json. This can be changed, but there are always some bugs that pop up when enough color combos are tried.
Assuming you wanted to add a detailed graphic of a house. At 32x32 pixels, you would have 1/16th the total viewable overmap of somebody displaying at 8x8. It would also require 16 times the processing power to scroll that map. (looks like the png is 20x20 pixel blocks)
Look at the trees, you cans see that by relation to the green tops, the trunks look colored, but in the actual png, they are just 1 shade off white. 256 colors here.
Assuming you added full 4 color blocks to the png, creating a nice colorful tileset, you would run into the issue of only having 256 tiles to work with. This is literally the problem they ran into with minecraft a few years back.
And just because something is tile based, doesn’t mean it is easy to run at higher graphical fidelity. Dwarf fortress the premiere ascii game frequently becomes unplayable after a certain amount of time in game due to the calculations requiring quite a few cpu cycles. This drops the game speed significantly. Call it frame rate death.
In summation, assuming you want to add detailed map icons, It will slow down the game, You will be limited in size, you cannot use any kind of aliasing or transparency, and you will give yourself a massive headache.
