What age group are you teaching? For adults, I think "less is more". For teenagers, something fresh to look at every so often is helpful - especially if it's something based on their interests. I see you are based in Russia, so I imagine it would be hard for you to get "English" posters for current films? Is there a cinema near you that shows films in English? If so, maybe they could help. If not, they could probably be ordered online for a small cost.
I also like to display student work on my walls - it can give them a sense of pride and achievement, it's cheap, and it motivates them to do well if they know their work will be visible.