Using substitutes early in development is normal, so that's really no problem. I think you would have been a little better off withholding on the creation of a post until you've got more information to provide on your project. Information such as your current progress, planned features, and information such as what tools you are using to develop the project (and your levels of experience with them) will go a way toward showing your capability to complete the project (and thus increase the likelihood that other members will follow your progress).
Unfortunately, I don't work on fan games anymore, so I don't have anything original to add as replacements. The placeholders you're using should suffice until you can either create the art on your own, or until your project generates enough interest to attract some of the talented sprite artists within the community.