This should do something like what you want.
# first line read images
# second line -- create black image size of sky.png
# third line -- create white image size of ball.jpg
# fourth and fifth line -- create mask image -- composite white image over center of larger black image and blur by sigma=15, then
# recomposite white image over center again to cover over blur inside the smaller area, leaving blur outside the smaller area
# sixth line -- add 25 pixel raised area in ball image
# seventh line -- composite raised ball image over mask image
# eighth line -- delete temps and swap last two image so mask is last, then composite using mask
convert sky.png ball.jpg \
\( -clone 0 -fill black -colorize 100% \) \
\( -clone 1 -fill white -colorize 100% \) \
\( -clone 2 -clone 3 -gravity center -compose over -composite -blur 0x15 \
-clone 3 -gravity center -compose over -composite \) \
\( -clone 1 -raise 25 \) \
\( -clone 4 -clone 5 -gravity center -compose over -composite \) \
-delete 1,2,3,5 +swap -compose over -composite \
sky_ball.jpg
50% reduced result:
Or if you want to control the darkeness of the shadow effect:
convert sky.png ball.jpg \
\( -clone 0 -fill black -colorize 100% \) \
\( -clone 1 -fill white -colorize 100% \) \
\( -clone 2 -clone 3 -gravity center -compose over -composite -blur 0x15 \
-clone 3 -gravity center -compose over -composite \) \
\( -clone 1 -raise 25 \) \
\( -clone 4
-evaluate multiply 0.5 -clone 5 -gravity center -compose over -composite \) \
-delete 1,2,3,5 +swap -compose over -composite \
sky_ball2.jpg
50% reduced size:
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