The Holstee Manifesto is a lovely script reminding readers of what is important in life.
The point of difference between a good school library and a great school library is the quality of the Teacher Librarians that work within it. Comparing them to google, is fraught with danger. I pinned this poster and then couldn't find the source. The text for this decal can be found on Phil Bradley's blog.
Walls that talk do a variety of decals and you can choose which colour you would like.
Syba Signs have a lovely word cloud listing key terms used when talking about inquiry research.
Etsy is a great place to go for decals as well. I love this Dr. Seuss quote to encouraging reading.
Being a crafter, I would coat them in some spray varnish (Matt) to stop those malicious students from peeling it off.
Of course, if none of these suit you, you can get your own made with your own mission or vision statement plastered across your walls to remind you, your staff and your teachers what your intent is.
Do you have any favourite decals for your library?