How to change default image

On lists, only the first 6 images show up as avaliable for the list thumbnail picture. Is there a way to get one of the other pictures from a list to be used as the thumbnail?

Thanks Maggie, it worked great :)

Also how do you change the default image for lists of lists, as currently it only shows the first image of the first list and there is no option to change it?

Another question for default images on lists, in lists of lists, there are four default images used, which are the first 4 images within the lists, how do you change these images and choose your own?