How did they get your files? That's going to be the first step.
If from a stock agency, you'd best take it up with them.
If from your own site, you may need an international copyright lawyer (very expensive) unless the offending site is in the same country as you. If the offender is in the US, send a DCMA. Otherwise you could write them a stiff letter asking them to take your files down, but not threatening to do anything you won't really do.