I solved that also:
I installed ImageMagick package (sudo apt-get install imagemagick) and I added changed base642png.sh:
#!/bin/sh
/bin/echo "$2" | base64 -d > /etc/openhab2/icons/classic/$1.png
/usr/bin/mogrify -adaptive-resize 80x80 -trim +repage /etc/openhab2/icons/classic/$1.png
So, the PNG gets resized and trimmed…
I looks quite alright…