Strictly speaking, if your designer has used a template image which they do have the rights to use (ie. creative commons or they have paid for the necessary rights) there is nothing to stop them taking an image from a place like freepick and amending each such image slightly. In this case, the first thing which you should do here is contact your seller and say something like:
I’ve noticed that you have used images from freepik in the designs which you just delivered. Please could you now provide me with evidence that you have the right to use these images commercially as I’m afraid that if not, I will have to report this matter to the relevant copyright holders.
If you do this, your seller should either supply you immediately with the relevant documentation. Alternatively, if they are a top seller and haven’t got their copyright in order, you will either scare them into buying the relevant licences and supplying you with them, or them offering to cancel and refund your order.
Ideally, they will have everything in order. However, if not, I would personally recommend that you then contact Fiverr Customer Support directly and lodge a copyright complaint.