Video from image with same image as background
Use

[0:v]split=2[vblur][vmain];\
[vblur]scale=640:360:force_original_aspect_ratio=increase,boxblur=5,crop=640:360[vblurred];\
[vmain]scale=640:360:force_original_aspect_ratio=decrease[vmain];\
[vblurred][vmain]overlay=(W-w)/2:(H-h)/2[out]