What are the WebM (VP9+opus) encoding settings compatible for YouTube import for a video with a still image?
Rather than framerate, the issue is there's only one frame. You'll have to loop the image.

ffmpeg -i Audio.opus -loop 1 -i image.png -pix_fmt yuv420p -ac 2 -s 640x360 -strict strict -r pal -shortest myAudioFile.webm



-loop 1 : infinitely loops over the same image
-shortest : stops encoding when the shortest input stream has entirely been encoded