A postcard can be a kept as a keepsake, something you keep for the sake of it. It can also serve as a memorial to holidays past, in which case you may call it a souvenir. Because "souvenir" is a French word, it makes the concept sound grand, but it really means the same thing as "memento" or "memory". Souvenir items are generally bought or stolen; a memento can be a more nebulous term, and is not always used to describe tangible items. A sexually transmitted disease, for example, can be a memento from a particularly enjoyable night out, whereas taking a souvenir from that same night out might involve borrowing an item of clothing.
A postcard is also used to convey a message; for this reason they are popular. There is a printed message on this postcard, in the form of a caption, which reads: "I greet from London". The caption is what makes the people in the photograph laugh, not to mention the situation of finding an outdoor postcard booth, in front of a museum, on the way to a pub. By the time the camera flashes, their laughs have settled into smiles. And by the time the postcards begin printing, the photographees are keen to get to said pub.
The six smiling figures all tell their own stories; that is what storytellers do.
In a few weeks from when this is taken, a photograph of the postcard will appear in a Montenegrin newspaper. The article will only have a loose connection to the postcard; it will be written by one of the members of the group being photographed. Others may lose theirs or give them away.
In additon to being a souvenir, the postcard could be described as a kitch (not kitsch) article: kitch as in "kitchen", because, that is where you will leave it until it slips behind the fridge. And the day you find it again, a few weeks later, you will remember (je souviens - from where we get "souvenir") that particularly good night out when someone thought it would be a good idea to use the photocard booth outside the British Museum on the way to the pub.