Pages/images/etc. are marking for deletion by adding the {{Delete}} template at the top of it. In the case of images, add the template to the image's page (i.e. the page you arrive at when you click on the image on an article). This template also allows you to add a reason why it should be deleted. Deletion can be discussed. If it is decided to keep the page/file, or if the reason why the page was listed for deletion is resolved, then the {{Delete}} can be removed from it.