[Nagí]

Hey! I'm updating this post to expose my advancement on the subject.

The previous answers have helped and learned me a lot. So I would therefore like to thank them once again.

At first, I obstinately continued in the way that has been suggested. I tried to fix the issue encountered with SetMask() while in combat. But I didn't found anything pertinent in the Blizzard interface.

So I decided to try as I thought with a folder containing my textures in order to avoid SetMask().
And I achieved to something working perfectly as expected. It's like this:

hooksecurefunc("UnitFramePortrait_Update", function(self)
    if self.portrait then
      if UnitIsPlayer(self.unit) then
        local race = ((select(2,UnitRace(self.unit))):lower());
        local gender = (UnitSex(self.unit) == 3);
        if gender then
          self.portrait:SetTexture(string.format("Interface\\AddOns\\RaceIconPortrait\\Textures\\%s-%s.tga", race, "female"))
        else
          self.portrait:SetTexture(string.format("Interface\\AddOns\\RaceIconPortrait\\Textures\\%s-%s.tga", race, "male"))          
        end
      end
      if UnitIsPlayer("target") then
        TargetFramePortrait:SetTexCoord(1,0,0,1)
      else
        TargetFramePortrait:SetTexCoord(0,1,0,1)
      end
      if UnitIsPlayer("focus") then
        FocusFramePortrait:SetTexCoord(1,0,0,1)
      else
        FocusFramePortrait:SetTexCoord(0,1,0,1)
      end
    end
end)

I'm conscious that it could be better, especially with the UnitSex() and the second %s (I have particularly struggled with this, for finally end up to this). Also I have add the functionality to reverse the portrait for the target and focus frame. If you have any suggestion I'll be glad to read them.