My best advice is to talk to the person that has the avatar and ask them to replace it with something else. If they're a jerk they either won't replace it or will replace it with something worse. But I think if the person is understanding enough, they would gladly replace it with something that wasn't so triggering for someone.

I had someone approach me on a site and they said although my avatar showed lots of emotion it was triggering to them. I didn't want to trigger anyone so I changed it immediately...I didn't wait a day or an hour or whatever...I changed it asap.

Don't be afraid to approach anyone if anything they use as an avatar or some other kind of graphic is offensive to you.