Hi @phelan, thanks for your update.
When you apply a background image to an element such as a slide but the slide has no other nested content, the background image does not give the slide any dimension.
If you set the slide to say 100% width and 100% height, then it may show if the slide is able to expand to 100% height and width of the slider height.
I see some slides there without any content, just a background image, so you will need to give some dimension like height or width, the auto style allows the slider to collapse on itself to a 0px by 0px div if there is no content or no dimension, and appears as if the image is missing on the page, because it is not visible at 0px by 0px.
Does this help?
At the moment the Webflow slider does not auto calculate for slide height, the slider will normally has as much height as the tallest slide in the set of slides in the slide mask.
There are custom script sliders that you can implement to a Webflow site like a Slick Slider which does do auto-height calculation for individual slides. If you search the forum for Slick slider that may be another potential option to achieve the goal.