Images for every post?

I’m using the Mu Theme.
Do I have to put in an image for every post? The default thumbnail shows up for all my posts. So do I have to go back and add an image or can I just remove them? I’ve tried disabling that but then they just show up with no image like it’s broken.

This is for our church web site. I want it to be as easy as possible. If someone ads something I don’t want them to have to put in an image everytime. Maybe we don’t have that many images.

thanks for any help.

Jim