The reason that the download was failing was not because the 4k size
didn't exist, but because the Artstation had no way to handle image
cover URLs. This caused it to pass nil to the download function.
Additionally, there was no way to get the preview URL size, i.e. the
smallest available image for an Arstation image URL.
- Adds support for cover URLs
- Adds support for preview URL size
2d88569fac