First I crawl my disk (about 2TB of references over the decades), then I search through my list of picture hosting sites, Pinterest does not belong to them because you seldom (almost never) find works posted by their creators but content of questionable origin or even without naming the origin - and that is a no-go!
The list is an unweighted collection of websites:
http://www.freestockphotos.biz/
This list is also found in the topic Art references sites or sources.
By the way, the sites I list above usually don’t need a registration (only a very few), and all are free of charge.
So I mainly rely on my memory of where I’ve saved something before I do a web search.
Michelist
Edited: For a more easy access to the list of websites I crawl.