Let me write a little guide how to create these backgrounds
>>2151674first we define the desired size and background color in ImageMagick
convert -size 1920x1080 xc:#0af background.png
convert -size 3840x2160 xc:#fca background.png
then we carefully select the cutest picture we have to paste on top of this background
to stick it on, use
convert background.png -gravity north cute.png -composite achtergrond.png
convert background.png -gravity south cute.png -composite achtergrond.png
play around with gravity or any other ImageMagick commands to get other effects if you desire.
here's another example.
