In the textures folder. The files all begin with Unit_Hud and end with the .tga file extension.

EDIT: If you're looking for the unit pictures which appear in the middle of the bottom bar when you have a unit selected then you'll also be interested in Unit_Picture.dds

If you open the Textures folder and scroll down to the files beginning with the letter 'U' then you should have everything you need and more.

If two mods used the same file name (I.e. they just editted the vanilla file), you're out of luck. You're going to need to do some photoshopping. 

I encourage modders to use unique file names for everything they can, just to avoid this sort of problem when merging mods. For example, in my Enhanced 4X mod, I put the characters "_E4X_" in every new file I add, both to prevent the kind of issue you ran into and so its obvious where the file originated from. Sadly of course not all mod makers are aware of this good practice or more likely they're mods were already pretty old by the time they heard of it, and didn't want to redo all their work.