When I post a url on Facebook to our class creator website, the preview photo it generates looks like a placeholder:

Is there a way to make it so that the preview shows the page that the url links to?

Ah! I'm so glad you asked this.

Facebook provides us some internal code we place on your site that tells Facebook what image to pull when you include a link to your web site. This code is on all home pages. In this case you elected to no longer route users to your home page, and instead route them to your What's New page upon login. That can make sense for many reasons and more and more people are making this decision. The problem is we don't have the code on the What's New page that tells Facebook to pull your site logo when you enter your web address in Facebook. We just need to put another copy of the code that's already on your home page on your What's New page as well. Submitting this to Programing. Should be in place shortly.

Thanks for calling this to our attention!

When adding a link to your site on Facebook, here is a tip regarding the two ways to provide a link and changing the image in a preview.
Sharing your site on Facebook

Gwen - the programmer has made the update to the What's New page.