Small notes for anyone being lazy, (Like me)
If you fit one of the dimensions it will center fine, so if you have a whole bunch of taller than wide photos, just scale to width and let the code center it.
You can also reduce the colors any way you want, and it will still work fine.
i:read["image"] card.image.map[() 1] card.image.paste[i .5*card.size-i.size]
Try importing random photos without doing any color reduction for neat effects