Forums is also a platform, where people publish all kinds of images. I think, it’ll be safe, if a person either noted it’s copyright free or you asked them just to make sure! There’s always human factor, so to be safe for sure, it’s best to double check, I think. Or if you can’t find something specific that you want - you can always create a thread on forums asking if someone can help. There are many nice people here 
P.S. A topic I was referring to:
(Sorry, if you didn’t want to be tagged).
These folks do some real nice editing job.
P. P. S. And here’s my own drive, to begin your collection with: EPISODE - Google Drive
There’s not much there yet, but hopefully will be more in future, ahah 