Should I trust the free photos are truly free?
You can trust that free photos are truly free, but be sure to check the licensing published for them on the stock site where you found them. If you're in doubt, download the free image and then upload it to the image search feature on advanced stock photo sites. If the uploaded image is flagged as one that they sell, then the image is likely a pirated copy. You should not use it if there's doubt that it is actually free of licensing restrictions.