Those are fine. The only "free" assets not allowed are assets that normally cost money, but are given away for free. And of course, not all free assets have the same licenses - some require attribution to the author(s) for you to legitimately use freely - so make sure to include such attributions in the game, if necessary.
From the rules on the Overview page:
Free assets only! Using paid assets is NOT allowed, but free assets and those under an open source licenses like CC0 are allowed. Note: this doesn't include paid assets that you got for free, only freely available assets are allowed.