Images missing (?) in Category post excerpts

http://kentelementary.ca/category/good-things-at-kent/

This is a link to a category of posts for a site that is up and running and just needs some finishing touches. My question is, why the little question marks – like there is a missing image or something in the excerpts. They are at the start of every post that actually has an image.

I am using a plugin called Thumbnail For Excerpts to make the images from the posts appear. Without that plugin, all I got were the question marks that indicate, I think, a missing image or link.

I would really like to clean this up.

Thanks for any help, Jacob