I work in a Public library and we are quite hot on copyright with regards to photocopying, scanning, journals and inter-library loans. We can also advise on plagiarism and methods of referencing sources. The whole area of media use, images, publicity and posters is a little more fuzzy. I can see that sharing knowledge with colleagues will be a good idea as we are being asked to do 'more of everything' rather than use our specialisms.
I am the main contributor to our library facebook page and can see that I need to be stricter with images sourced to advertise our classes and events. Simply sharing other posts should not be an issue as they are attributable. Finding sites such as Flickr, Opsound & Pixabay is really helpful, and I found the following link worth investigating for more Creative Commons sources for images; http://wptavern.com/13-sources-for-free-public-domain-and-cc0-licensed-images
Here are two pictures that I sourced from Flickr which are creative commons, and also which I have hopefully attributed appropriately. ;
Creative Commons 20091204_Hermitage_library by Christopher John SSFB is licensed by CC BY 2.0
Christopher John SSFBy: Christopher John SSF
Library, Hermitage of st Bernardine, Stroud, NSW
Creative Commons Matcha Cafe by Hoàng Lãng TửBy: Hoàng Lãng Tử is licensed by CC BY 2.0