Keyword Optimization

[SEO]

Hi... I have been working hard trying to improve serps for my site. 4 weeks ago, I was on page 15 for keyword nature wallpapers, two weeks later, I made it to page 1 (10th position). Everything was going good, but suddenly, my serps start to fall. I don't even get any visits from that keyword. Now I am on page 6 on google. So, what do you think I can do to improve my site for "nature wallpapers"? Here is the site: Nature Wallpapers (http://pc-nature.com) Thanks a lot!