Jacob, here is a new thread to follow on from the discussion here.
... but there is no text on the slides, just a background image. It just doesn't work unless I pick from those which are listed under one of the purposes. I personally can't get text to render on 'any' media as you indicate.
Specific examples of this problem would be very helpful. Do you mind starting a new thread to chat about this issue?
Here is a screenshot of a set of slide backgrounds with the title image chosen from a 'collection', which works fine.

Here is the same document with the verse slide background changed to an image selected from All > Image (i.e. from Kind = image).

This problem occurs on any image used for heading, text, or Bible verse, if not chosen from one which is listed under 'Purpose'.
Yet, even if I choose from some of the 'reference' purpose type images, the text does not overlay either, as below:

In fact, I cannot get it to show the text on 'any' slide unless I select a new 'title' slide image background from one of the 22 collections. When I do that, I can get the Bible verse to appear on a slide. In the screenshot below, the text of John 1:1 does not appear when the image background is chosen from a 'collection', but the next reference John 3:16 does work after I changed the Title background to the same image.

Effectively, I can only write sermons if I choose from the 22 image collections under purpose. Since I only like a few of the designs, if I were to start using sermon editor on Sundays, I would have to use the same few image backgrounds again every few weeks. That's a shame, especially since the media tool gives the impression that there are thousands of images for me to choose from for use in sermons. I'm not sure how many of the problems I have encountered are limitations by design, or bugs which need sorted out. Hopefully the choice will be greater soon, either by design change or bug fix.