Rather than framerate, the issue is there's only 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

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