How to assign SharinPix licenses to Salesforce users?

1. From Setup, enter Installed Packages in the Quick Find box, then select Installed Packages.
2. Click Manage Licenses next to the package "ImageManagementBySharinPix"
3. Add or Remove the licenses corresponding to specific users

Learn more from Salesforce documentation

https://help.salesforce.com/articleView?id=distribution_managing_licenses.htm&r=https%3A%2F%2Fwww.google.fr%2F&type=5