By default the following image is displayed for a project:
You can change this default image by adding in the same project folder a
.png image with the exact project
name and extension. Example, if the project is called
montpellier.qgs you must add an image named
montpellier.qgs.png. Note that the image has the project extension too.
It might be needed to clear the cache and force reload the page from the web browser.