The idea exchange icon, when sharing via Facebook or Google etc, is by Creative Stall, available via the Noun Project, license CC-BY 3.0, here: https://thenounproject.com/search/?q=idea exchange&i=995966
Edited a bit: moved the arrows, removed some light from the lightbulbs so it now instead looks like so:
The clock icon (on the homepage) was made by Alexander Wiefel at The Noun Project
https://thenounproject.com/alexander.wiefel/collection/timing/?i=61159, licensed under CC BY 3.0 US.
The background image on the homepage is from https://www.rawpixel.com, licnese: https://www.rawpixel.com/free-image-license: "You can use any of the free images that you download from www.rawpixel.com for personal and commercial purposes. However, the image copyright is still the property of Rawpixel, so you cannot resell, redistribute or claim the image as your own." (as of 2017-06-20)