stock photo images

When building a site i use royalty free stock images just like most other people, Yes i buy them, yes i keep a copy of the licance etc,etc.

But the images are not just on one stock photo suppliers but several sites.

There are also places on the net that provide stock images from the stock sites in bulk that you can download (possibly ilegaly if used ).

I recently found that the images i had paid for were available on four of these sites where you can down load them freely.

My question is this....

If someone uses the images they downloaded for free in a site/s design that they intend to sell or use personly how would you know. the images from the stock suppliers are downloaded all over the net by hundreds if not thousands of people worldwide.

I understand that the images from the sites you payed for can be downloaded and used over and over again by those who purchase them, but again is there a way of telling if the supplier/producer of a site has paid for the images they have used, Yes you can send them an e-mail asking them to provide proof of the licance but as we all know your computer can fail, get viruses so on & so on, so what if this happens and you no longer have the licance ?.

Again going back to the original question can you tell or not ? and if so what you do about it ?.