Please read this journal from our friend !EveLivesey (https://www.deviantart.com/evelivesey)
I must just add this precision: You may use stocks from flickr, but you MUST choose pictures selected into the "Creative-Commons" and "Modifications allowed" folder. Then, you MUST give the name of their owner. This rule is available for all the stocks you choose: always control the licence of the pictures you use and always credit their owner.
And, of course, YOU CANNOT REPACKAGE THEM AND REDISTRIBUTE THEM AS STOCK: it is a theft!
Some useful links: