Попробуйте выполнить следующий сценарий bash:
#!/bin/bash
max_width=0
max_height=0
border=10
# Trim all files to remove the white borders
for i in *.png; do
convert $i -trim "${i%.*}"__trimmed.png
done
# Find the max width and height
for i in *__trimmed.png; do
w="$(identify -format "%w" $i)"
h="$(identify -format "%h" $i)"
if (( $w > $max_width )); then max_width=$w; fi;
if (( $h > $max_height )); then max_height=$h; fi;
done
# Add a small border (optional)
max_width=$(($max_width+$border))
max_height=$(($max_height+$border))
# Add borders to all pictures so that they all have the same size
# "-gravity center" will center them
# -background None will avoid the default white background as your sample image
# was a png with a transparent backgroud
for i in *__trimmed.png; do
convert $i -background None -gravity center -extent "${max_width}x${max_height}" "${i%__trimmed.*}".png
done
rm -f *__trimmed.png