That is not what you showed in your first post. There, you showed the built-in obituary notice with weapon icon. That is not done with a hud element. It is hard-coded into the COD engine and cannot be edited. The only way to flip the kill icon is to edit the weapon file as I illustrated in my post.
As for the hud element you've just posted, just edit the IWI file for the thompson - which is hud_thompson.iwi - by converting it to a .DDS file and flip it around in your favourite image editing program. You will have to do this for every weapon image.