Found possible image thief on Pixiv.

Thought I found a good trove of undiscovered images, but then when I popped each one into the upload bookmarklet, every image was not only already on Danbooru, they were all from different artists, too.

I had already made an artist entry for him because the one image I started with wasn't on danbooru. I only found out after I started noticing the other images that this one image had already been uploaded to yande.re, which is why I know who ACTUALLY drew it (the actual artist had deleted the original in the mean time, however).

What is my responsibility and/or course of action in this case?

Delete the artist entry and report the account to Pixiv.

Thanks! Done the first, will attempt the second.

Also submitted a report too, linking the sources of the stolen art for one of their multi-image posts.