I'd tread pretty carefully here. If you paid $2000 for the photo, you might want to check to see if it came with a licensing agreement.
One way to view this is through the lens of privacy. The Restatement (Second) of Torts § 652A subjects privacy invaders to liability for the resulting harm to the interests of the other. Because you mention public personas and your facts don't involve disclosing private details, negative publicity, or interfering with seclusion, one might think appropriation of name or likeness applies.
Restatement (Second) of Torts § 652C
One who appropriates to his own use or benefit the name or likeness of another is subject to liability to the other for invasion of his privacy.
Comment (a) to this section explains that a person's identity is in the nature of a property right. Courts have tended to recognize an individual's interest in the exclusive use of his or her own identity, going so far as to require licensing for usage.
The most common way to violate this "property right" is by appropriating someone's name or likeness to promote a business or product. Cases abound on the subject, but a classic one is White v. Samsung Electronics America, Inc., 971 F.2d 1395 (9th Cir. 1992). To summarize, Samsung ran a picture-based ad featuring a futuristic robot dressed in an evening gown and turning "Wheel of Fortune"-styled letters. Vanna White was nonplussed and brought suit, which she won on appeal. In its opinion, the Ninth Circuit argued the operative question wasn't how Samsung appropriated her likeness, but whether they had done so. The point of citing White here is simply to observe that a person's "likeness" probably extends even further than mere photo reproduction.
Back to your facts: posting actual images of celebrities for a commercial purpose appears a much more clear cut appropriation of likeness than White. While it may seem similar, I'd distinguish your situation from that of restaurant "Walls of Fame"---which showcase signed photos of celebrities with the owner---in that the celebrities you mention weren't using your product or service at the time of the photograph.
You might argue the photos aren't being used for advertising, but the plaintiff would counter that you posted them on a client-facing business website. In the Ninth Circuit this would be a question of whether there was an appropriation, not the way in which it was accomplished.
For argument's sake, assume the court agrees your usage isn't commercial. Even that doesn't necessarily mean you're in the clear. In § 652C, and varying state-by-state, non-commercial purposes are also subject to scrutiny:
Comment (b). Apart from statute, however, the rule stated is not limited to commercial appropriation. It applies also when the defendant makes use of the plaintiff's name or likeness for his own purposes and benefit, even though the use is not a commercial one, and even though the benefit sought to be obtained is not a pecuniary one. Statutes in some states have, however, limited the liability to commercial uses of the name or likeness.
It's important to remember the Restatement isn't binding: it simply attempts to "restate" what courts (which are binding) have had to say on the topic. If you paid a lot of money for the photo, it might pay to see if it came with a licensing agreement.