Retouching portfolio using tutorial images - Copyright concern

Is it possible to use images from tutorials for retouching portfolio (even writing photographer's name)???.
For examplie buying (or download from here) a FStoppers' Elia Locardi landscape tutorial or a Dani Diamond portrait shots, edit the images and display them in my online portfolio?
Or again: RGGEDU tutorials don't have any pdf license about retouching but I'm paying to get the course along with the images to be retouched.

I would like to use them to showcase my skills as a retoucher but I've never worked in this field so I can't get these kind of professional photographs, I also don't have any photography studios.
But now I'm feeling skilled enough to work on professional images and these tutorial photographs would perfectly show my skills. I take and edit my own photos, but the photos themselves are not at a professional level (light setup, pose...) so they wold describe me simply as an amateur... But on the other side I feel very confident with my retouching skills.


A note:
In the United States, the Copyright Act defines "derivative work" in 17 U.S.C. § 101:
A “derivative work” is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a “derivative work”.

Thanks!