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?

This is a tricky one. It is best to build a bottom up forecast. You need to internally decide your sales strategy and make your best estimate of how many customers you will sell, how much you will sell to them and how much repeat business you expect to have. You will do this for each market you will sell to. I would be as granular as possible when building this internal forecast. If you have specific target customers that is great include them. Combine all of this for the next 3 years and you have your forecast. It will be wrong by the way, but it's all you have to go on.

The real problem is when present this to investors. There is a tendency to give too many details and for some people to get caught up in the details. I would provide potential investors only the high level projections and explain the process you used to derive the numbers. Once you dive down into the details of a plan like this you are inviting all sorts of criticism that you really cannot defend, then it can turn into a game of gotcha.

Any good investor will know that the projections are wrong but should appreciate the process that you have used to create the projections. At the end of the day they will either think that what you have created is a good representations of your product/service potential or not. They will not benefit from getting too caught up in the detail and you certainly will not. Keep it simple and try to steer any conversations back up to the substance of the projection rather than the details.

I've done many revenue projections for investors in my startups. As long as you have a methodology and rationalization for your projections you will be fine. So pick the more conservative number, assume a 5% market penetration, and explain what the numbers will look like.

One of the most crucial tasks an entrepreneur has is to calculate the size of their market, and the potential value that market has for their start-up business. Determining the market size is critical. It tells you and your partners, team and investors how much potential business is really out there. Market size becomes far more important if you ever need to raise funding for your business. It is one of the most basic digits every potential angel and VC investor is going to expect. To calculate your market size, you will either be looking for data on the number of potential customers, or number of transactions each year.
There are a variety of ways to acquire this data. Census and labour bureau hold a lot of information, and most industries have formal associations which compile and track this type of data. Once you have the data you want to make sure that you are presenting it in a powerful way in your pitch deck since it is one of the most important slides. Thiel includes not one, but two slides around the market and its size. Market size, or the number of potential customers or unit sales is one thing. You need to know how much revenue that market has to offer.
Realistically, no start-up should or can expect to gain 100% market share. Trying to capture an entire market, without first targeting several niches, price points, customer sizes or geo areas for roll out, is going to be financial suicide for most entrepreneurs. That is about $1B a year from just one extra revenue stream, at just over 18% of the available market share. Of course, most new start-ups cannot expect to even command that much market share.
Even if you could, most seasoned investors will not believe it until you prove it. If you have no idea what’s a reasonable amount of market share in your industry, Projection Hub says one hack is to anonymously call around to all of your local competitors and find out how much volume they are doing. Also factor in the static versus evolving marketplace. Do not forget to factor in your own impact on the market. Their price cutting also slashed the value of the market in a huge way.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath