Your logo's border looks particularly thick. It doesn't look very good on such a minimalist site.
"Vivapixel helps you store and share your photos and videos !"
There shouldn't be a space before the exclamation point. Furthermore, there shouldn't be an exclamation point at all. You're not making us excited right now, you're selling your product. It looks better if you're confident enough to stick to a period.
The checkboxes look very generic. Why not use a font-specific checkbox, if you need checks at all?
Bold fonts to me suggest slovenliness. Any reason you can't use a normal font thickness, and experiment with sizes to emphasize?
Work on the quality of your writing. If you've got three bullet points, it helps if they're about equal in length. "Crystal clear photos & videos" is overlong and doesn't say anything: aren't all photos crystal clear online? Does crystal clear video mean HD? (I'd also tell you to hyphenate crystal-clear, but it would be better to go with another phrase entirely.) "Safe and Fast" should see the f in "Fast" lowercased. Furthermore, these aren't points you should be making. I don't think of photo sites as unsafe or slow. You bringing these up makes me wonder just how fast and safe you are. (It's like the proverbial restaurant that advertises "Our meat doesn't come from cats.")
I notice all the photos up front were taken by Dave. Any chance of getting other photographers/some spoof accounts to look better? Also, for the love of Pete remove the border from that photo. Nothing adds to slightly disorienting clutter than vaguely noticeable borders around things that should remain borderless.
The "create new account" button isn't clickable. Make it so I can click that button.
Don't give us a tour using thumbnails on the front page. That's cluttered. If you had more going on, thumbnails would be fine, but as it is it looks pretty cluttered. Also, work on the spacing. If you've got to have thumbnails, give them some space. Don't jam text up against the photo. Again, work on spacing (between commas in the second thumbnail) and punctuation (the first thumbnail has a sentence that doesn't end in a period) and capitalization (a lot of offenders here).
There are a few other things - the spacing of the top links, the links in the footer - but those are minor compared to your main problem, which is that this splash page doesn't make me want to sign up. I don't see the advantage, beyond having a cool web site URL, to using this above Flickr.
Remember that a good design means convincing people they want to use you. Right now, that's not happening, and that's a problem.