While I am searching for pictures of brains (so far without success) I was thinking that others might be looking for sources of pictures they can use as well.
Best of free stock photos
Here are my picture resources, in no particular order:
I would describe this site as artsy. Wonderful pictures, some very professional looking.
2. Refe Real Life Photos
The name says it all...
Free and premium membership.
Another huge collection. Not all pictures look professional. Lots of "people" photos.
Images, vectors, and videos.
6. Death to the stock photo
Lovable, artsy, different.
7. Little visuals
Very interesting pictures. Great for articles.
With search engine.
Very many professional looking pictures
11. IM creator
Curated. Also includes templates, icons ...
12. New Old Pictures
Millions of creatives
Smaller but nice collection. More than pictures.
15. Free High Resolution Photos
Receive 20 pictures per email when you sign up and more every Sunday
By far the biggest site on this list. You can also sell your own pictures on there. Free and paid pictures (subscription).