An anniversary gift from the whole family
Children remember the middle decades; grandchildren remember the later ones. Ask each generation for the photograph of the two of them from their own years.
Ask for one photograph with both of them in it, from the years each person actually knew them. That second clause is what makes a whole-family anniversary different from any other group gift.
Two honorees, and three different marriages
A marriage looks different depending on when you arrived in it.
Their children remember the middle decades — the working years, the house that was always half-finished, two people who were often tired. The grandchildren remember a couple who had already been married thirty years by the time they turned up, and who were, by then, quite good at it.
Both are accurate. A whole-family piece is the only version of this gift where you can hold more than one of them at once, and that is the reason to do it this way rather than picking a single image and asking everyone to agree.
Skip the wedding portrait
There is nearly always one formal wedding photograph, and it is nearly always the first suggestion. It is a fine picture and it is the weakest choice here, for a plain reason: almost nobody being asked to contribute was alive when it was taken.
The photographs worth having are later and less arranged — a kitchen, a car park, a folding chair on a beach in 1987. One of them will turn out to be a picture only one branch of the family has ever seen, and that is usually the one.
One person does the chasing
Collecting from relatives is the hard part of any group gift, and an anniversary doubles the constraint, because a photograph of only one of them is a photograph of a person rather than of a marriage.
Say that in the ask. One picture with both of them in it, and a line about what was happening. The narrower request gets answered faster than the open one, and the guide to getting relatives to send photographs covers how to phrase it so it can be dealt with from a phone.
Leave the number off
A fiftieth is a number, and a piece carrying it is dated to the week it was given. The pictures and what people wrote about them still make sense at the sixtieth, which is the whole test.
Signature is sized for a piece several generations are putting their names to.