An illustration that I posted is being used by another user as their profile graphic or background without my permission.

If a profile graphic or background that you posted to pixiv has been redistributed or manipulated without your permission, please report this to us by using the inquiry form below. Enter the content ID (URL) where your content is being posted and the content ID (URL) where you originally posted your content and then submit the form. We will investigate your claim and handle the matter.
Depending on whether we receive a report from the rights holder on pixiv and are able to confirm the claim, we will shut down the offending account.

pixiv inquiry form