There is no verb in the sentence. If we add 'tomo/hizo' (took), we get:
Quien (tomo/hizo) las fotos de la fiesta = Who (took)the photos of the party
los fotos = the photos
"¿Quién va a la fiesta?" means "Who is going to the party?"
"De quien" means from who?; "De quien(?)-whose(?)";
It means, "For whom is the party?"
"Con quien estas" translates to "who are you with" in English.
"De quien" means "from whom"
"Hola quien eres" translates to "hello who are you" in English.
"Quien cumple años" in English means "who is having a birthday."
"Y esa quien esss" in English translates to "And who is that?"
Con quien means "with whom"
"Quien habla contigo" in English translates to "who is speaking with you?"
Translated to English from its Spanish origin, fotos de caballos means "photos of horses". There are thousands of photos of horses online to look at.