Collage shortcuts

Is it defendable using pictures made by others? Luckily, legislation defines the final art work as creation without any obligation to its sources. So don't bother about it, just go
My own justification is the transitory character of the pictures which are my raw material. There is a growing tendency to protect everything in order to obtain royalties, but I am against it. Art cannot exist without freedom and without exchanges between artists – dead or alive. Bach is a famous example for his “borrowings” (his transcriptions), Picasso for his variations on famous paintings. Or in more recent times L.Berio and his musical collages.
Another question concerns meme hacking. For me, perverting a work of art shows its stereotypes or refreshes the work which is weakened by overreproduction. Anyway, the original remains visible in the new collage but gains a new meaning. Here it is the perversion of a painting celebrating the king of France.

Another example is my former streetwork, when I was working on billboards in Geneva/
This work was very exciting and was not motived by political reasons, but for sport and fun. I loved the work on big sized posters. I learned by this work to let my works go, as they remained on the board for a couple of days only.As for the pictures displayed on my site, they are free of copyright.Because I want to share them. Their resolution is unsufficient to make orginal-like reproductions. I hate art sites with watermarks on the pictures. Either you show them or you don't.