This can be as simple as including a textfile as an attachment to your post, or just copy-pasting the license text, or even including a pic like this:

Otherwise, projects that try to respect rights and licensing can't really use them comfortably, especially when you're not around to say "Yeah, i gave permission."
I'd suggest CC-0, since the Creative Commons licenses are pretty much universal for art and music, and the other CC licenses impose a variety of restrictions on how the work can be used.
It's the art equivalent of the permissive free-and-open-source licenses like MIT and freeBSD.
Mod Note
Global Forum Rules have been updated for clarity of this matter. Send a PM to Forum Admins if you have any concerns about this.
Global Forum Rules wrote:Asset Licensing
Unless otherwise stated, it will be assumed any user-made assets (i.e. maps, sprites) posted here has an implicit license of Creative Commons Attribution-ShareAlike 4.0 International License. If you want your assets to be licensed differently then you must attach a license to your post;
Creative Common's website lists their attributed licenses and public domain licenses. See their License Chooser if you're not sure what to pick.