I like the picture frame idea! You can also try something like a large cork board - I used to love those when I was growing up, pin up pictures and things from different events etc.

Is the window used alot? You can also try something out of the box by doing a curtain rod across the entire back of that wall, sheer if you want natural light in or a thicker fabric to keep light out

Is that wall the only possible orientation for the beds? Wondering about placing the tall dresser where the bed on the left is sited to help balance the window - if there is another wall with enough width for two twins.