Like most Larkin pieces, there's a lot going on here. Let's start with a quick summary of the action and the conceit. Larkin has been granted permission by a lady he knows well, a lover probably, to finally look at her photo album. He pages through it, innocently at first, making superficial remarks, but he quickly moves on to more significant contemplation.
He explores the nature of photography itself and wonders if his emotional response is tethered to the art form before second-guessing himself and attributing the past to his suddenly complicated response to a simple photo album. He finds himself mourning the younger lady in the album as he steals one picture for a keepsake.
Key elements of the poem are his apostrophe to photography itself and his sudden mourning of the girl in the photographs:
But o, photography! as no art is,
Faithful and disappointing! ...what grace
your candour thus confers upon her face!
How overwhelmingly persuades
That this is a real girl in a real place.
The photograph makes this girl more vulnerable and human than a painting or poem would. This realization, though he knows the camera lies and encourages false narratives about the past, causes him to treat the girl in the photograph as someone deceased. By the final stanza, his tone, which has ranged from whimsical to sarcastic and inquisitive, has turned defiantly elegiac.
In short, a past that no one now can share,
No matter whose your future; calm and dry,
It holds you like a heaven, and you lie
Unvariably lovely there,
Smaller and clearer as the years go by.
Note the double meaning of lie. He has determined the camera lies, but he literally refers to her figure in a photograph.
In your analysis, be sure to note the ABBAB rhyme scheme of every stanza and the loose iambic pentameter that grants this poem a formality and seriousness that is required by the poem's end. See link for a recording of Larkin reciting this poem.