I feel serendipitously placed to respond here, because I'm sitting in bed beside my fiancé, a widower, looking at a photo on the wall of him his wife kissing. I was just thinking about the issues raised by the OP when I opened Mumsnet for some distraction!
I'm also well-qualified, in a sense, because I'm a therapist with some training in bereavement counselling. (I say in a sense, because therapists are just messy humans and because we're all individuals with different feelings, all valid and worthy of respect.)
And it's difficult.
I feel relaxed and comfortable with these pictures in my fiancé's home, near his bed even, but I still feel that third presence and sometimes feel like the visitor I still am. When we move in together I'd rather our shared home is decorated for the two of us. We've talked a bit about having separate areas in the new home for our memories, other areas for creating new ones. We'll need to talk more, keep being open with one another and listening to each other's needs.
It took us both some time. We were both lucky to have some therapy and supportive friends while we navigated our feelings together.
I disagree with previous posters criticising the OP's partner's feelings. It's natural to feel insecure when the man you love loves someone else; that she has died doesn't make that go away. I've struggled with it, as many others do also.
It's hard to understand how it is possible to love two romantic partners at the same time if you're monogamous and haven't experienced anything like it. It was hard for my fiancé to understand and come to terms with loving someone new, too. Being able to place new photos among his old ones is a sign he has room for a new life; it's been gradual and this way new love is more secure, because it isn't pushing out the past prematurely. In that sense, keeping photos of his wife is a sign that he is serious about his relationship with me and will not rush it.
I'm wondering if the OP and his partner have talked about this, as it might help her to see where the OP has come from, through his grief, and where he is now.
There aren't any children involved in our situation. I agree it's very important that their wishes are taken into account and also that a new partner isn't seen as removing pictures of their mother.
I'd suggest openness, allowing your partner to feel she can express any insecurities, perhaps aiming at accepting that this is a stage and a process you can support each other in, exploring possibilities together.