TL;DR: The traditional motivational poster is kind of hit or miss in value. IMHO, you're better off using the wall space for functional things that help you get stuff done or for showcasing your business's achievements and dreams.
I like the other answers that are on here, but I wanted to add a few other thoughts as well.
First, I want to say something about traditional motivational posters. You seem to be at least hinting at those in your question. I think for most people, they aren't going to make much of a difference. Motivation is far more complex than a pretty picture, and if you're trying to motivate yourself or your team where motivation is lacking, hanging up posters like this isn't going to fix it. You need to get to the heart of the issue and fix that.
To make things worse, for many people, motivational posters are insulting. You hear phrases like, "Management thinks hanging up some pretty pictures with cutesy quotes is going to change anything? Those guys just don't get it." It's because of this that the whole world of demotivational posters arose, as Nishank mentioned.
I might suggest two broad categories of things that are better to put on your wall than motivational posters.
First, give the best wall space to things that help you get stuff done:
Basically, with this point I'm saying to turn the place with everything you need to get the work done. Things that help you get stuff done should get the best spots on the wall.
The second category that I would suggest is things that are about your company's past, present, and future, even if they don't serve a direct purpose. For instance:
For me, having things like this that are specific to my business are far more motivating than a poster or some artwork would be. (That's not to say there isn't room for some good quality art or even traditional motivational posters, but that those should have a lower priority than the above.)
Society6 - A great selection of posters but could take time finding ones you want.
Blik - These aren't posters but could drastically spice up your walls.
We have demotivational posters around our little office that always get a good laugh. Got them from Despair.com.
I think the workplace should really reflect the culture you're trying to create.
Some funny ones: