I am really sorry to hear about the loss of your grandmother - and the worry over your grandfather who has early stages of dementia.
It is in the very early days of bereavement for everyone in your family, and, as you say, after such a long marriage, it will take your grandfather a long time to come to terms with his grief.
I am of an age now where I have seen many people, including myself and my family, dealing with grief. I have to say that what your grandad is doing is no different to what many others do.
A friend of mine lost her husband 4yrs ago and her home is almost like a shrine to him with photos every which way you look, and which she regularly says a few words to. My mum kept a photo of my dad by her bedside to which she would say good morning, goodnight, I miss you, I love you etc each day/night for 21 years until she died. A friend of mine, who has autism, lost his mum earlier this year and regularly moves a framed photo of his mum from room to room so she can be with him. He talks to her and kisses the photo too. None of the above behaviours are/were due to dementia - it is simply ways of coping with loss.
I lost my husband 5yrs ago. I don't talk to photos around the house but I do keep a photo of him on my phone as a screensaver. I also still talk to him in my head and sometimes even say a few words out loud to him - and I probably aways will. It brings me comfort.
I could go on, I know many more who are going through these behaviours after loss.
Nobody can say what the long term effects of loss will be on your grandad and his mental health, but at this stage I would say just let him process things in his own way. I would only become concerned if it stopped him being able to move on with day to day life - such as becoming tied to the house, not eating/drinking, actively searching for her etc.
I wish you all peace and strength to get through this tough time.
