Funding is all about trust.
My experience, we have people willing to seed-fund us only based on idea alone (except, we have worked together before). The investor said it bluntly, "I am funding you, not your idea". There is that trust involved that we are smart enough and won't mess up his money for no reason.
Having said that, we lose our interest for the investment. At that early stage, we knew we haven't proved most of our assumptions in real world. There are many things that might go wrong and i feel bad having to use that money just to prove the initial model. After all, i rarely see business plan work as how it is written.
That is good problem to have. If you haven't got that level of trust with someone or having a rich uncle (family, friends and foes), then your best bet is probably to get someone to do the screen-shot, do a cheap advertising campaign (Google Adwords / Facebook Ads) and prove that there is a traction to your idea.
Try to use oDesk, 99designs etc to do the screenshots. It should be affordable.
If you can prove that people are coming to your site, and clicking 'Sign up' or 'Register' even before you build the back-end, that means, you have something that people want. It's easier to convince investor with all those data available than just a 'plan' that full of assumptions.
If my explanation is unclear, maybe you want to do some reading about 'Lean Startup' or 'Minimum Viable Product'.
Hope that helps.