i would suggest to make it very clear on the first page that the wallpapers are free to download as that is a key feature. the site looks very nice, only stopped by quickly but the quality of teh wallpapers was very good. liked the anime and comic ones. SEO may need some attention eg "anime girl in the sky" - i doubt would draw anyone to your site from a search engine. The girl there is hatsune miku and that too wouldnt bring people in as it is so frequently searched . My suggestion would be that if you include ability for people to comment on the wallpapers then the seo should follow as users would input content / detail like this. As your revenue stream seems to be low margin advertising then SEO will be critical to make good money. let me finish by saying its a very nice looking site - good work indeed.
nice, but i think the pict is too small.. i think better if you make it larger and how about remove left or right sidebar when they open the pict...