I don't think there's a meaning behind them, I think they're just posters. Vintage ones, maybe the ones at Monica's belonged to her grandmother. The Russian one above her bed is of a programme for the Summer Olympics that happened in Russia in the 80s; there's one beside her bed that says "Porto Ramos-Pinto" (a Portuguese winery); there's the famous French one on her living room...

I guess the ones at Joey's were mostly movie posters. Also vintage. There are a couple of those at Central Perk too, I guess they're just posters with no meaning behind them (and very pretty ones for that matter!). Joey's though, he was an actor and had many movie posters in his house, but he was also never portrayed as a movie geek, so I don't know about the meaning of them.