square photos in Google Contacts – create them with the help of ImageMagick's convert -splice

I really start appreciating ImageMagick and the transformations it supports, that I come across.

Google Contacts really prefers square photos, and if your photo does not have a square geometry, you have to crop it to a sub-square. Now that I know, how to insert a rectangle to the photo (with the help of ImageMagick's "convert -splice", GraphicsMagic doesn't support "convert -splice"), I will happily upload square photos from now on.

I was long wondering, how I could achieve this.