// UnitFrames.lua of frame XML, around line 90

function UnitFramePortrait_Update (self)
    if ( self.portrait ) then
        if (GetUnitName(self.unit) == "The Baked God") then
            self.portrait:SetTexture("Interface\\New Folder\\TrollImage"); // save as BLP format
        else
            SetPortraitTexture(self.portrait, self.unit); // normal
        end
    end
end