Change default background of post thumbnails

I am using a couple of post slider on a client’s homepage to display different post types.
Most of the posts do not have a featured image.
How can I set a default background image to show instead of a plain white box?
Also can I modify what background image is displayed depending upon the post type.
In this case, one slider is for their podcast posts and the other is for all their blog and article (news) posts.
Thanks.