Adsense vs Wallpaper Site?

I have a wallpaper site, with 200 unique visitors per day, but I can get only $1.5 (25 click).

If with this site with your optimization, how much you can earn?

Well thats not about SE optimisation but more an ad optimisation, no?
Did you ever try to change place, color, size of you ads?

If that's your site, how much you can earn?

I dont know much about the behaviour of wallpaper interested people because I do not look for wallpaper and I dont own such a site.
If your strategy does not work try another one. Thats why I asked if you ever changed colors, sizes or place of your ads. Maybe AdSense might not even be the right advertiser for your site but some affiliate programm offering photo service or something. Just try it.

My site is: www.GirlOnDesktop.com, yesterday, i had 500 visitor, with 4000 pages impression, but earn only $0.9

Can you give me some advices?

Really there's only two ways to increase your earnings: more traffic and (as -mj- has already suggested) changes to ad placement/style.

Your main problem is that being a wallpaper site most of your ads are going to be for other wallpaper sites. Non of these are going to be prepared to spend large amounts per click, so your earning per click are going to be low.
Additionally, people are likely to only click on these ads if they can't find a suitable wallpaper on your site. If your range of wallpapers is small or of low quality you increase your click through rates, but reduce your traffic long term. Catch22.

You're never going to make much from Adsense on a wallpaper site unless you have huge traffic volumes.

besides optimising the adsence placement you can also try writing a few lines of description for each wallpaper where you include some other words that might trigger ads for different subjects. These ads might have offer a better CPC to you but in this case you will need to observe if your CTR is going down ( because most of your traffic is there for wallpapers) Adsense optimization is all about experiment so unless you experiment you will not learn.

Thats not a bad conversion rate. I'd leave it alone and look at finding more traffic.

I have just built a car wallpaper and hope this will help me earn more by some car keywords relate!