I do have some sympathy for you, OP.
Sometimes it is difficult enough not to be jealous of previous, live girlfriends, if you know that they meant a great deal to your present partner and you''re not feeling very secure in your own relationship yet.
Ex-partners who have died (especially young and tragically) can arouse very difficult feelings as, firstly, the relationship was broken up forcibly and involuntarily, and not because either party wanted out, which must sometimes make you wonder if he would still prefer to be with her, if he could. Where someone had lost a previous partner involuntarily, I can see how it could seem that you will feel in permanent "competition" with her unrless your partner assures you otherwise. (the Mrs De Winter syndrome) There is also sometimes a tendency to idealise the dead, especially if they died young, and so you may fear, not only comparison, but comparison with someone whose best qualities have been immortalised and lesser ones forgotten. So I do think it is a difficult situation for you and I do sympathise,
I think the answer here is time. If you have been with your partner for six months and his previous girlfriend died only last year, it is not surprising for you to wonder if he is ready for a new commitment. If you really care about him - as you seem to - I would try really hard not to impose any conditions for another six months or so. At that point, you will have been together for a year and he may well be ready to put the photo away himself - or be willing to, if you ask him sensitively.
His memories of her will never die, of course, but he may not need a prominently visible reminder of her, which is what is upsetting you.