If you're not using RPG Maker, then you need to buy the assets here on itch.io where they have an open license. The ones sold via Steam have a special discount because the license is limited to RPGMaker only. Thanks!
I believe I purchased your asset pack in a bundle on humble bundle. Was that a limited license for RPGMaker.
If you got it in a Humble Bundle, then you have the general license and there are no restrictions. :)