All questions regarding mapmaking for Heroes of Might and Magic 3: The Shadow of Death.

Here, I've extracted this article. I've never done it myself. And even though it mentions WoG, as I understand it, you just need to place the portraits in the Data folder, and instead of the original portrait of the corresponding hero in the game, yours will be displayed. Some maps (Xiedu, Ira the Mage) include their own portraits in the archives, you can take a look.