You already know the answer, which is probably why you are asking someone else.
The honest version is this: if you would be uncomfortable with your partner doing the exact same thing, writing letters to someone they used to be with, sending pictures, keeping in touch in ways they have not mentioned, then you already have your answer about whether it is wrong. The test is not complicated.
That said, situations are rarely that simple. The depth of what you had with this person matters. The reason you want to reach out matters. There is a meaningful difference between wanting to send someone in a hard situation something to read because you have a good heart, and wanting to reopen something that still has a pull on you. Only you know which one this actually is, and it is worth being honest with yourself about that before you pick up a pen.
People inside genuinely benefit from outside contact. A letter, some reading material, knowing someone out there is thinking about them, those things matter more than most people realize. If that is purely what this is, it might be something you can be transparent with your partner about. If transparency feels impossible, that tells you something too.
Whatever you decide, go into it clear-eyed about your motives and what you are willing to own if it comes to light. Things have a way of coming to light.
Keep us posted on what you decide.