Huaban and Dutang are pin boards like Pinterest. If you mean free to use as in copyright, that's a murky issue as they don't really say. Probably they are copyrighted but maybe not? Like Pinterest people can post anything from anywhere. A lot of the art for the covers I did have appeared on...
Photoshop. Also Sketchbook, which is free now.
There's also a free online photoshop clone called Photopea. It's the same layout as photoshop, only thing is it gets laggy since it's an online app.