No Explicit License for Images in "assests" directory

@RichardLitt and @daviddias contributed all the images in the `assests` directory. Based off of `LICENSING_POLICY.md`, images and docs should be licensed under Creative Commons Attribution 4.0 International (CC 4.0), but there isn't an explicit license for the art besides an MIT license, which typically isn't meant for non-code. I'd like to seek approval from both @RichardLitt and @daviddias to license their images under CC 4.0.

Some Background:
I was thinking about a logo for my project, [Geo25rey/ipmail: InterPlanetary Mail - Decentralized Email alternative using IPFS](https://github.com/Geo25rey/ipmail), and I was thinking about incorporating the IPFS logo as part of my design. Is that allowed under CC 4.0?