"Small modifications" are considered "derivative works" and won't necessarily let you avoid licensing issues, but you're getting into the world where you'd be better off talking to a lawyer. Freepik.com doesn't appear to really be free - you have to subscribe in order to get "commercial" licenses, and even then you can't be sure that freepik.com actually has the license they claim to have for the image. I'd consider pixabay to be a bit more reputable, but you run the same risk there. The point is anytime you use someone else's content you are taking a risk. The least risky approach is to only use your own original content. What I do personally - on the advice of my own lawyer - is subscribe to istockphoto.com. If I can't create an illustration on my own, I'll use one from there where I can prove I have a legitimate license for it. For really specialized things that don't have stock images, I'll sometimes use content from Wikimedia Commons after being very careful to comply with any usage and attribution requirements listed there.
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