When it is labeled as CC0, it should be a CC0 license. Make a print screen of it and save it.
I'm not sure yet how to do it. First I would need a bigger 'following', and I would need to get in touch with my followers who would need images for their posts on Steemit. Maybe I will create a discord channel for those who would like to use my images. With a list of needed images I could search my database for already existing images and upload them in a post free to use for up-voters only. Since I am new to the platform, it would take a while to get noticed ;-).
Another option is to sell images under single user licenses for crypto, until you get bigger. We all get paid in crypto, so it's a lot easier than converting.
There might also be stock image sites that take crypto, but I'm not sure if there are any.
Great idea!