It's common for established websites to get some amount of spammy backlinks. They might be linking to random images on your site. There are lots of bots that link out indiscriminately for many reasons. It might not necessarily be a negative SEO attack on your site. They might be doing other sorts of manipulative link building, and are just linking to your site in order to cover their tracks.
Whether you should be concerned really depends on things like the size of your site, the velocity with which the bad links are growing, whether the bad links seem to be a coordinated effort, and other factors. If you feel like you need more assistance on this, you should work with a reputable SEO consultant or agency to do a more detailed audit.