Sudden spike in backlinks - should we disavow?

[Adab1]

A kitchen remodelling firm in Australia has noticed a big increase in spammy backlinks to their website since April this year. Majestic shows that referring domains and backlinks during that period have gone up from 400 domains (10,000 links) to 1,070 domains (47,000+ links). About 100 are sitewide links.

The vast majority are "follow" links directed at image files on the site - ending in .jpg. Ahrefs now shows the number one anchor text (23%) is a period (full stop) "." Most of the links come from .us domains, eg:

cowboysr.us
blackphoto.us
alldpic.com

Google Search Console isn't showing any of these links, there's no penalty on the site and there's been no noticeable change in rankings (if anything organic clicks went up over May, June, July) ...  so we're wondering what action - if any - should be taken.

Are these links likely to have a negative impact on the site and homepage? Should we disavow these links?

Appreciate any advice. Thanks.

[Igor.Go]

Even if flow metrics on those links are good, the sudden spike is already a red flag for search engines.
If those links aren't organic get rid of them.
I'd get a list of referring domains and bulk check the domain metrics (age, pr, trust flow and etc) and if there's nothing good - get rid of them.

[Adab1]

Thanks Igor - none of these sites look legit. All have low to non-existent metrics like trust flow. We'll go through the list and disavow.

Any thoughts on what the purpose of these scraped image links are - is this a form of negative SEO?