Add 12px margin around the main screenshot image

That way it'll look much better when the screenshot itself doesn't
have its own margin.

Closes #83
Closes !512
2 jobs for 83-weird-gray-box-behind-every-screenshot in 4 minutes and 49 seconds
Status Job ID Name Coverage
  Build
passed #905121
debian-stable-x86_64

00:01:07

passed #905120
fedora-x86_64

00:04:49