Your designer must use properly licensed art. I only use royalty-free art. Nothing like getting a huge bill from Getty Images for using unlicensed art! I know. It happened to me once when I paid someone to make a book trailer for me. Long story short, I learned that whoever posts the artwork on the Internet is liable if the art was not properly licensed with purchase. Since we’re talking about your book and you will be the one posting it on your website and such places, if the artwork is not properly licensed and you are ‘discovered’ by the roaming Internet robots, you will be left holding the legal bags. My suggestion is that you purchase your own art (the designer can send you the links and tell you what size to buy) or only use reputable designers who live in your country so that you can take legal action against the designer if the copyright holder ever comes looking for you.