Is it permissible to work on designing images to be sold as NFTs, and to receive profits from a client who distributes "credit points" to his customers by freezing a portion of capital in accounts that yield usurious benefits, and is it necessary to verify the source of others' earnings?

There is nothing that necessitates the prohibition of the questioner's work, as long as the images are permissible, because the ruling on the questioner's sale does not change based on how the buyer subsequently sells the images. Every soul is responsible for its own earnings. Giving points to buyers is permissible, and its ruling is that of promotional prizes and gifts. As for the design of digital images, they do not fall under the category of prohibited images.