try this
# first line gets size of person.png image
# second line reads bg.png image
# third line resizes persone.png image and brightens it using -contrast-stretch (you can also try -brightness-contrast)
# fourth line creates a radial-gradient and decreases the effective radius using -level
# fifth line puts the radial gradient into the alpha channel of the resized person.png image to achieve a vignette effect with transparency
# sixth line composites the modified person image over the background and adjusts its position relative to the center
dim=`convert person.png -format "%wx%h" info:`
convert bg.png \
\( \( person.png -resize 90% -contrast-stretch 0,5% \) \
\( -size $dim radial-gradient: -level 10,100% \) \
-compose copy_opacity -composite \) \
-gravity center -geometry -5-5 -compose over -composite person_on_bg.png
