Why does anyone need to add the dimensions? Surely the build process can do this ahead of time or the server can do it on the fly. It only needs to be thought of once.
You still need to either use a fully integrated dev/build system which allows it, or explicitly add the placeholders. The build process of course could add the actual numbers, but I don't think I've ever seen that happen outside of CMS content. (Specifically content, not even the theme / static stuff)