Yes, they take lots of pix - this department makes more money than the bars or the casino, so the pix aren't cheap.
However, if you want a pic (or pix) of your entire family together, you might want to do it on one of the formal nights because they'll pose you just right.
As to waiting to choose other pix - you can, however, by the end of the cruise you'll have to go through boxes of pix to find the ones just of your family.
Can't tell you exactly the cost of the prints on the Mariner - but might be around $15 and up - someone else can give you better idea.
On the other hand, you might consider buying a digital camera - even a onetime disposal one and take your own. Most pharmacies (CVS, Walgreens, etc.) will print out the pix when you get home for a lot less than what you'd pay on the ship.