Be aware of the license, even for the "free".
Some "free" open-source license require you to make your own software open-source too, some other just ask you for some credits and mentioning the license. And in some cases it's free for anything.
Opengameart.org has some licenses explained
https://opengameart.org/content/faq#q-proprietary
So when you're planning to go live with things, make sure you know what art-work you got from what resource and what license it is.
Just to make sure you won't get into trouble in future.