I wouldn't worry, to be perfectly honest. There are different things governing you as the site owner versus the poster. My solicitor has outlined some of these aspects to me for my own site due to DMCA's and legal issues due to members posting things that aren't known to me as infringing... and basically, if you take action when outlined to you, you are not in the crapper. The member can be... You can be if you did nothing, OR, if you were intentionally running some site allowing lots of illegal image posting and distribution or such copyright work... but a member finding some image online and sharing it as part of their post is their problem legally if pursued. Pass their details and let lawyers pursue them via ISP's and such.
To cover your backside, just make sure your legal policy outlines something to that affect. The default Xenforo one already covers this out of the box... you just need to keep something similar in your own:
You agree to not use the Service to submit or link to any Content which is defamatory, abusive, hateful, threatening, spam or spam-like, likely to offend, contains adult or objectionable content, contains personal information of others, risks copyright infringement, encourages unlawful activity, or otherwise violates any laws.
I've had to remind my users of copyright quite a bit... and Google images doesn't make life any easier. People think because they find it in Google images it's a free for all in posting it anywhere online.
I am in Australia, yet my server is US... so I have certain obligations under both jurisdictions my solicitor pointed out to me. The web is complicating things with this ease of International access, hosting and obligations nowadays.