What would be a good quote from Frankenstein or Shelley about family, love, etc. to serve as a caption for this picture?
This is a great picture for a lot of reasons, and of course it would be possible to attach all kinds of captions to it. Connecting it to Mary Shelley or to Frankenstein might prove to be a bit more challenging.
You mention the possible themes of life, family, love, and inspiration. These are not, of course, the first things one thinks of when reading Frankenstein; however, certainly some of the themes from the novel are connected to these ideas. Unfortunately, most of them are framed in a negative light/
In the first chapter of the novel, Victor talks about his childhood. He says:
The innocent and helpless creature bestowed on them by heaven, whom to bring up to good, and whose future lot it was in their hands to direct to happiness or misery, according as they fulfilled their duties towards me.
This quote suggests that it is the parents' responsibility to bring their children up, and they can do so by creating misery or happiness. It looks to me like the father in this picture has chosen happiness. It’s a possibility. He also says:
. . . the companions of our childhood always possess a certain power over our minds which hardly any later friend can obtain.
This, too, is a possibility.
It is not surprising that most of the positive or inspirational quotes in this novel come from people other than the monster, as we know the kind of sad, angry, and miserable life he lived. Everything he says about family and friends is said with bitterness and longing rather than joy and love. So, we have to look to someone else to supply the positive quote/caption for which you are looking. Even the demon man he meets in chapter ten has more hope than the monster:
Life, although it may only be an accumulation of anguish, is dear to me, and I will defend it.
The best I have to offer comes from a young servant girl. In chapter eight, Justine Moritz says:
Live, and be happy, and make others so.
It is an ironic statement, to be sure, as she ends up dying because of misplaced love and devotion to the Frankensteins (and her religion, of course).
If you are interested in being a little funny or sarcastic, you could always insert some awful, depressing quote from the monster and then add “Not!” to it. Most of the published quotes from Mary Shelley are no more helpful, unfortunately. This is not a "love and happiness" novel or author.
Seems to me the best fit is Justine’s line. It captures the essence of a father talking to his sons, giving them his fatherly advice. In every way, this is just the opposite of what the monster had, but it is exactly what this photograph depicts.