Ask HN: Stock image as website logo: any side effect?

I'm building a website. It is bigger than just a side project. Found an image on one of the stock image sites, bought it for $20. I use this image as site a logo. I contacted the artist. Asked him to make few illustrations for my website in the same style. He did. Everything fine so far.

No I'm thinking about uniqueness of this logo image. The picture was sold for a long time on this (and few others) stock image sites. It is still available there.

Should I get (buy) some exclusive rights ? If I do this — what happens to the people who bought this image before? Should I just add the unregistered trademark™ symbol somewhere near this image and site name? Should I ask the artist to modify the image to make it unique? (something like adding one or two letters from the site name, "pied piper" style, you know))