Motivational posters? Really? I just sort of thought those sprouted like mushrooms in offices, I never thought they actually got intentionally purchased.
Instead of presenting options to your team, present the problem.
"Hey I noticed the dev office is looking pretty sparse. What do y'all think? I have a budget of $$insert money$$ that could be used"
It may be that they don't want decorations and, instead, want an arcade cabinet to duke out Pull Request opinions over a game of Gauntlet. Maybe they want a big TV they can stream starcraft matches on. Maybe they want a big graphic wall wrap of their team/org name. Maybe they want a soda fountain. Maybe they want the whole room done up in whiteboards (That's what I would want). Maybe the light from the windows causes massive glare and they just want some gosh darned curtains.
The point is - you don't know what they want or what would actually help the team. If you are willing or your company is willing to spend a little bit of time or money to make it a nice office for the team, give them the opportunity to tell you. Start with a meeting that is framed with the question "If we could do anything to this room to improve it for y'all, what would it be?" Then, with some of those ideas, start breaking it down into budgets. "Well, replacing the floor with one of those big novelty pianos is beyond what we can do right now but you request for hammocks is totally doable..."
TL;DR - Ask your devs. They have different needs in their workspace than you do. They know what challenges they face in their current space and can offer both legitimate and ridiculous options for resolving those challenges and making it a nicer space.
EDIT: With the additional information you've provided. First - get them the boring, functional things they've requested. In addition to more light/a bookshelf ask if they want any particular books. The rest depends almost entirely on your budget. If you have the money and are considering getting a bookshelf to match any new decor - bring someone in and have a professional plan the room. If you don't have the money and want something more akin to 'hanging posters' then find out what favorite genres or movies everyone has and get some cool posters. Additionally(or alternatively) find out if they're happy with the whiteboard space, I for one would be thrilled to work in a room with wall to wall, floor to ceiling whiteboards.