We made a similar (albeit genuine) mistake a while back. A temp image used for design mock ups was so similar to the rights free one supplied by the client that it slipped through the net and onto the live website.
Turns out, it was an image owned by Getty Images, who are notorious for taking companies to the cleaners over copyright infringement.
Entirely our own fault, and we held out hands up for it but it still resulted in me signing a cheque for several times what a days photography would have cost.
A hard lesson learned that day.
The fault/blame, legally so ands with the client – not the design agency, we just took it on the chin because it WAS our fault.
Personally, I’d be chasing the cheeky sods for a small fortune given the competitor circumstances involved here. Get your client involved too!