Example sites
Ling's Cars
techno-kitsch
Free Photos
Copyright Free Photos
galleries, no search, download, no attribution
Flickr
all creative commons license search, check specifics for attribution, etc.
FreeImages
mingled with IStock Getty, OK
Google Images
labeled for reuse, modification, even commercially search
ImageAfter
very categorized
Morguefile
modify or credit, Flickr Creative Commmons search engine
The Open Photo Project
great photos, see attribution
Stock Free Images
required account and attribution
Unsplash
requires download, optional attribution badge, Trello use
Photo Tools
Pexels