To mark a page/image/etc for deletion, simply add the {{delete}} template to it. In the case of images/videos, simply add the template to the image's page (i.e. the page you arrive at when you click on the image on an article). See for further details on using the template. If a file/page is marked for deletion, the issue can of course be discussed. Do this either on the talk page of the article/file. 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.