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