Default locations for images/code

Given the presentation structure, it is reasonable to make the code look for unfound images in `images` and code in `code` :)
I'd actually be inclined to make a separate loop at the top that looks for candidate locations (each calling `is_file_path?`), and then just have the two branches--found a file and didn't.

That both avoid the duplication, and allows for things like configuration adding other code directories which I've considered.