Because it's not at all deliberate that the pics are inside the frame that way. And you definitely can't include your own layout function like even BA does.
There isn't any real reason to need
two images where we could just use one.
Moreover, sticking one or more elements like that into the game sounds like a great way to wake up and find stuff's broken down the road, due to further hacking on that area.
If you want a "framed" look, then use that look for the unit picture. If you want to do some fancy Lua stuff over the top of that, then do that. We only ever needed one image there.
It's not a big deal to me, personally. I'm going to release a full Windows installer that includes that fix, and other customizations. I just don't like that it won't match the Linux builds in terms of behavior.