yes that's a 3d model, and its left of centre, not bottom left - the minimap is bottom left!
the only way to do it is to open either war3.mpq or war3x.mpq and search for the 3d portrait model of the unit portrait you want to use. All portraits have the same model path as the unit itself, with a "_portrait" added to it.
e.g.
if the footman model path is units\human\footman.mdx
the footman's portrait model path is units\human\footman_portrait.mdx
so extract the one you want, and import it into your map using the import manager, changing the model name to whatever you want.
e.g. if you want the archer to have the footman's portrait, extract the footman portrait from the mpq files and change its model name to
units\human\archer_portrait.mdx
and before you ask me where the mpq files are or how to open them, they are in your warcraft 3 folder, and you open them using WinMPQ, which is linked on our links page
<====== click "Tools and Utils" over there