where is your style.css file saved in your app directory? It may also have something to do with the ? at the beginning of your image file path, that seems out of place
The style.css file should be placed in a www folder that lives at the same level as your app.R file. You can move the image into this folder as well and update the path accordingly.
Also, I had to add a dashboardSidebar() between the header and body: