itch.io

Yes, all assets that are included can be used in things you make (unless I explicitly say they can't be, in the documentation for a particular project). The only thing you're not allowed to do is to resell them as an asset pack, so using them in a game is allowed.

Credit is optional (I'm not expecting most players of a game to be interested in the code or stock assets that were used to make it... :P) but if you want to credit me I'd recommend a standard CC-BY 3.0-style credits tag ("ASSET NAME by CREATOR NAME [newline] <URL to where you got it from>", with the <>'s actually being there in order to denote the start and end of the URL).