We would urge you to choose a license while sharing your artwork here on krita-artists.org. Even if it is proprietary license please mention that below the image. For other licenses you can go here and choose a license. For example If I choose my artwork to be under this license. I would write it as “License: CC BY 4.0” under my artwork.
Not writing anything under your artwork would mean that it is proprietary and nobody can use or share it. Which is also fine.
If you work is a collaboration or remix of somebody others work, please provide proper attribution and credits to the collaborator or author.
If you want to keep the license as proprietary want to allow Krita Foundation to use it for Promotion of Krita on Krita website with attribution to you, you should specify that under the artwork.
Soon we will be creating tags for various CC-BY licenses so that you can tag your artworks accordingly.
Any suggestions and advice are welcome from the community.