Don't be sorry for being new, were it not for newbies we'd all be sitting around, patting each other on the back, and playing HoI1 for the gazillionth time.
When you say "unit pictures" I think you mean what the old hands call icons, and these can be found in the gfx/interface/models folder. They're listed by country tag (e.g. 'eng' for England - which is the playname for UK, 'saf' for South Africa, etc.) and model number. The model numbers correspond to the different unit types available and you can get more detail on these in the db/units/divisions folder. For your purposes, though, I think all you probably need to do is find the unit icon you want to replace, make a note of its full title, rename it (say, by adding 'old' to the title - this way you can retrieve it if things go wrong), copy the eng version of that model, paste the copy into the models folder, then rename it with the original title of the icon you're replacing.
Hope that helps. If it doesn't there'll be plenty of more expert modders dropping by to help.
