É uma questão de quanta mágica / tempo é derramada neles.
Os retratos são encantados e alguns são ensinados a agir dessa maneira. Da própria JK Rowling, via Pottermore:
When a magical portrait is taken, the witch or wizard artist will naturally use enchantments to ensure that the painting will be able to move in the usual way. The portrait will be able to use some of the subject’s favourite phrases and imitate their general demeanour. Thus, Sir Cadogan’s portrait is forever challenging people to a fight, falling off its horse and behaving in a fairly unbalanced way, which is how the subject appeared to the poor wizard who had to paint him, while the portrait of the Fat Lady continues to indulge her love of good food, drink and tip-top security long after her living model passed away.
Alguns retratos têm muito mais trabalho que outros.
Some magical portraits are capable of considerably more interaction with the living world. Traditionally, a headmaster or headmistress is painted before their death. Once the portrait is completed, the headmaster or headmistress in question keeps it under lock and key, regularly visiting it in its cupboard (if so desired) to teach it to act and behave exactly like themselves, and imparting all kinds of useful memories and pieces of knowledge that may then be shared through the centuries with their successors in office.
As fotografias gerais não são colocadas através de magia e trabalho tão extensos. Como na vida real, uma foto é tirada em um segundo, mas uma pintura a óleo leva muito mais tempo.
Fotografias gerais são feitas para se mover com a Solução em Desenvolvimento . Ainda mágico, mas não é tão mágico como os retratos. Novamente, da própria Rowling :
Q: Why did Colin Creevey's camera work etc?
A: As a vast number of people have pointed out to me in the last twenty four hours (some of them related to me by ties of blood) Colin DID develop a photograph from his camera in 'Chamber of Secrets' (my previous answer stated that he never did so).
Cameras, like radios (or, as the wizards call them 'wirelesses' – they're always a bit behind the times when it comes to Muggle technology) do exist in the wizarding world (there's a radio in the Weasleys' kitchen and we know there are cameras because of the moving photographs you see everywhere). Wizards do not need electricity to make these things work; they function by magic, but in the case of such objects the wizards liked the Muggle invention enough to appropriate the idea without adding cumbersome plugs/batteries.
I have an old notebook in which it says dev sol (potion) magic [indecipherable word] photos move. Adept as I am at interpreting my old scribbles, I can tell you that the original idea was that wizards would use a magical developing potion to make their photographs move.
Colin é o problema da foto em Câmara Secreta
Os jornais funcionam como as fotografias e os posters desejados, com o mesmo tipo de solução em desenvolvimento, poção ou encantamentos menores.
É tudo a mesma coisa, apenas em diferentes escalas de magia.