I found a small bug in the original game. I can't fix it myself – it doesn't work, so I'll suggest it to you.
Basically, when you give neutral creatures an external dwelling in the map editor, but don't set the 128x128 building icon, the game shows a white square instead of the 64x64 building icon in the castle's hiring menu. I haven't checked, but I think the same will happen if you disable the 128x128 icon for standard dwellings. That is, the logic is: "128x128 building icon => 64x64 icon in the castle," if the first one is missing, the second one is also missing, and I don't know how to make it so that if the 128x128 icon is missing, the 64x64 neutral icon is automatically set.
I've looked through the resources a bit. As far as I understand, the settings for displaying images are located here: \data\UI\HireCreatures\HireWindow\Item\Dwelling. I couldn't find anything else; this Indian code and the logic of those Indians who wrote this game are driving me crazy, help me fix this bug.