Backlinks with jpeg/pdf

I don't know why but some domains have a ton of jpeg backlinks referral domains. Are these backlinks worth it to index?

These may be backlinks from '-k.html' pages that there dozens of threads about:
https://www.blackhatworld.com/seo/wtf-are-these-spammy-links-ending-in-k-html.1492143/
https://www.blackhatworld.com/seo/n...em-ending-with-k-html-any-experience.1472230/
https://www.blackhatworld.com/seo/h...cklinks-with-k-at-the-end-of-the-url.1459441/
https://www.blackhatworld.com/seo/how-to-avoid-auto-k-backlink.1562700/
https://www.blackhatworld.com/seo/negative-seo-image-linking-issue.1538098/
etc

From my own observations, sites that are ranking already tend to get these backlinks from having their content scraped. They're not a reason for them ranking in the first place though.