Wordpress code required that will allow automatic attachement of featured image to posts

My wordpress website creates posts automatically from various incoming RSS feeds (text only, no images). I need to make my website automatically attach a featured image to each post (this has to be done manually in wordpress currently). I want to be able to say, for example, here is image #1 and i want this image attached as a featured image to all posts created from RSS Feed#1. I want image#2 attached to all posts from RSS Feed#2 and so on. The posts are assigned to "categories" so one way migh be for example to say attach image#1 to all posts that are assigned to category name "XXXX" and image#2 to all posts assigned to category name "YYYY" and so on.