Hi Eugene,
Doodlekit runs on a different backend technology than PHP and we don't use flat files - everything is database driven. So uploading your own HTML page is not possible.
However, if you are a website designer, you should be able to use our CSS override to do most any design you want :). I use it all the time. The advanced CSS editor is in the Admin Tool under Advanced Settings. You can upload images you want to use in your design there and reference them through the code.