Great article. I've been meaning to do something on this myself, after climbing through the photography submitted on steemit and realizing it's usually not marked for license either way..
https://search.creativecommons.org/
has about 10 sites to search, and if you search on Bing for "creative commons images" it's pretty amazing how many different sites come up.
just mentioned u in my latest post when ur mentioning of the communities thing liek subreddits the new feature, its gonanbe amazing gonan raise steem price up past $2!