What that is is one of the reasons I don't use any AV - they are garbage (false positives, cpu overhead, overly aggressive heuristics, they flag and quarantine useful tools and malware samples on my machine unless I can whitelist those areas, etc)
That warning is most likely because someone has a link to an image in their signature that is hosted on media.bigoo.ws, and Avast has blacklisted everything coming from that domain.
Post a permalink to that exact post in your screenshot above and I'll tell you what's there.
