Best export settings for still image with audio
To minimise the file size for distribution of something like this, you need a Variable Frame Rate encoder, which will effectively just encode 1 frame, and set the frame rate such that there's no further frames. I've not seen this available in Adobe (or any commercial product I've looked into), but definitely there are services I've seen encoding video this way - I've seen examples of very small Facebook video files that appear to be VFR and done this way. In the absence of finding an available encoder, I'd just lower the bitrate as much as you can in Adobe that the image is still good, and use that. The upload time to YouTube might suck, but it only has to be uploaded once anyway.
If anyone does know of an available VFR (Variable Frame Rate) encoder, please post the info - this isn't the first time I've seen the question asked, but I'm yet to learn of any end-user available encoders to do this.