Following might be the reasons for this site showing higher in search results:
1) This site may be getting lot of back-links from other sites.
2) As per the recent Google Panda update, if one site is linking to another sites, which can be useful for the viewers, then Google happens to give edge to that very site ( In this case it is there).
3) Last but the most important factor.
Google shows personalized search result ie, if you have surfed/visited the site previously also and downloaded some image from the very site, which means you have performed "CALL TO ACTION".
As you have performed Call To Action on the site.
Google takes the site as 'familiar to you' and shows the site as your personalized search result.
Other people may not see the site on same position as you are seeing it for a particular keyword, all because of "personalized search result" theory.
To see the de-personalized search result use the following:
replace &q=wallpaper with