Nothing wrong with using placeholder graphics until you build the engine. A lot of more experienced people do the same thing so you can work out the entire play mechanics before making everything look pretty.
Also, sometimes the simplest ideas are often the best...
Remember, if you need help with something, or want to get an idea of how something might work if you are stuck figuring things out... ask someone! Everyone here pretty much is generally very helpful and will be more than happy to point you in the right direction or even post examples.
Welcome to the boards!