I took a quick look at 500px who reported 1.5-million users in 2012 (source: Techcrunch http://techcrunch.com/2012/11/28/gorgeous-photos-in-your-pocket-500px-arrives-on-iphone/) with 10% as professional photographers. The article also mentions monthly growth of ~100,000 users. If your targeting the same market as 500px, use their reported numbers to back-up your assumptions. I'm sure there are other services who target the same user community - I'd spend some time collecting this type of data.

Is your service designed only for professionals, or would you see a semi-professional (part-time) and advanced home-user?