Unfortunately, there are no guarantees. There is no way to verify that a piece of art submitted by someone will not be found in court to have infringed on someone's copyright. One reason is that two works can be judged to be similar because they reflect similar themes and ideas of artistic expression, without any actual copying, so you just have to take your chances with a jury. Another is that the contractor might be less than honest, and there's no way to prevent that. A standard approach to infringement matters is to include an indemnification clause in your contract, which means that if the contractor infringes, they have to defend you against the accusation, which will reduce to suing the contractor and hoping to mitigate the damage to you by using his pockets. Hopefully he has money.
Professional liability insurance would be a wiser approach (in addition, not instead). Although you would / could be sued, the insurance company should take care of the matter for you (though you might have to sue them to force them to do so). One reason why an insurance company might refuse to cover you would be if it has sufficient reason to believe that you violated a term in the policy requiring you to not "contribute" to the problem. So if you have reason to think that a work is infringing but you ignore that evidence, then you would have negligently contributed to the problem, which could relieve them of their duty to take care of you. That would all get sorted out in a suit against the insurance company (which you want to avoid). It is not enough to declare that there is obviously no intent to infringe: what did you do to show that there is no intent to infringe? This is pretty much where the legal theory stuff ends. If you can figure out a way to match the submitted work against protected candidates (e.g. "other paintings of George Clooney"), vigorous efforts to weed out infringement would be the first line of defense against an infringement suit.
