GIMP is suitable for preparing derivatives of the public-domain Art in this batch: preserve an untouched source file, work in an editable project, then export a correctly sized JPEG, PNG, or WebP.
The official GIMP FAQ states that GIMP is distributed under GNU GPL v3 or later, may be used for any purpose including commercial work, and does not restrict the kind of work you produce with it. GPL obligations concern distribution of GIMP or modified GIMP software; they do not automatically license exported artwork.
Use the official downloads page and official documentation. Before installation, verify the current system requirements and the published SHA-256 checksum for your installer on the downloads page.