Hey @amos! This isn't a direct answer to your question, unfortunately, but how married are you to using ioslides? When Yihui talks about some of his reasons for preferring remark.js and xaringan over competing RMarkdown slide formats, he mentions customisability and specifically names a workaround for your problem: including a background-image on a slide.
https://www.jaredlander.com/2017/07/fullscreen-background-images-in-ioslides-presentations/
So it's possible, according to this workaround, but it involves dropping JavaScript into your slides (and bringing jQuery in), which is pretty frustrating.
Having played with xaringan a little, I think you'll find it much better when you want to heavily customise the slides with CSS.
In particular, with xaringan you can apply CSS classes or properties directly in the RMarkdown like:
---
background-image: url(https://github.com/yihui/xaringan/releases/download/v0.0.2/karl-moustache.jpg)
background-position: 50% 50%
class: center, bottom, inverse
# You only live once!
---
These styles aren't quite applied to the top-level slide element (remark.js uses CSS transform to scale your xaringan slides according to window size, so there are some wrapper parents), but it's still applied above the slide content level, so it should suit your requirements 