You can pre-purchase whatever packages are available for your cruise in the Gifts & Amenities section of the
DCL website.
Yes, you can get your photos taken wherever you see a photographer. However, you have no control over what pictures or in what sizes those pictures will appear in your folder later. The pictures that appear can be bought (individually or using credits from your package) as-is or left behind.
My family didn't take any, but I saw folks getting formal pictures taken in one of two spots: either in front of the Donald statue at the base of the staircase in the atrium, or in front of a white background off in a side alcove of the atrium. I assume that white background was filled in by Shutters with some kind of scenic background before being printed.