General

1) Were I you, I wouldn't have posted pictures of her without permission.

2) IMO, the most you should do is encourage until she gives you a clue that she wants to gain.

3) I would complement her without saying specifically it was because she's fatter. I would center as many activities around eating as possible. And I would make sure whatever food she eats / has available to her is delicious. Better food costs more, but you are trying to link eating with positive feelings.

this. even though the pics aren't even about the same person lol.

You are very lucky, she's looks to be adorable. I would be a subtle encourager. Be sure house/apartment are well stocked, frequent eating out opportunities and always, always, complements on her appearance.
I would not even mention weight or weight gains, just that she looks amazing to you.

I am so sorry that you’ve lost him.

Make sure you are open about your intentions. Don't be sneaky. It is her body, and you need to be there for it as it changes. Her mobility and possibly energy will decrease.

You're in a relationship. If you want her to gain weight, have a discussion. Let her know about your kink. She might be willing to engage with it or even enjoy it. But respect her decision most of all.