You mean if the owner of the copyrighted images finds out that his copyrighted material has been used without his permission?
Who gets in trouble then?
Maybe he doesn't do anything about it.
Or maybe he sues everybody involved.
If you have no money, he might not do anything.
If it is clear that a company with actual assets used his material, or if he's just feeling protective of his property, then he could go after you. He could contact you with a nice letter to ask for $10.00.
Or he could file a lawsuit to reclaim the damage you did to his brand to the tune of $3 million.
The LEAST likely person he would go after: the illustrator.
I had read an article. i.e. if I give you images that are copyrighted ultimately it is YOUR responsibility. Yes I stole them, but you accepted stolen goods.
If the owner finds out he will go after you. I can always claim that YOU told me to give him whatever images I could find. You have no interest in paying for them. Most likely the owner can send a take down notice - in web business the hosting companies are not very kind. They would rather get rid of you than have you contnue using their services for few hundred bucks a year.
I would say it's a good idea to put it in your contract "All illustrations submitted will be original illustrations and are free from copyright restrictions" or something along those lines.
You can also do an image matching search on google to see any similar images or exact copies. This should be a regular thing you do when accepting work that is supposedly original, just drop the image in the google image match search and google will show you if there are any other images like it or exactly the same. It's not perfect but at least you could have a record that shows you attempted to see if the image is copyright or not.