Setting featured blog thumbnails

Hi - As far as I understand it, the thumbnail that appears for a blog on the home page is taken from the featured image for that blog's latest post.

My question is - is there any way to set a fixed thumbnail for a particular blog rather than the blogger having to set a featured image every time they write a new post? At the moment, if they write a post that doesn't have a featured image, their blog thumbnail returns to the default thumbnail (at least as far as I can tell that seems to be how it works). Is there any way around this? For example, it would be great if there was a setting in each individual blog's admin menu where they could 'set featured thumbnail for homepage' or something...