As far as I know, you can't.
So you have two options. Either change your resolution to 1024x768. Or just use that photo as said size on your 1440x900 resolution.
BTW, I just used a photo of mine at 1024x768 and changed my resolution to 1440 x 900 and it's not blurry to the point, that you should be bothered to make it clearer.How to I make a blurry picture clear but not change its resolution ?
You can't! Not the answer you were looking for... try and see if they offer it in any other resolutions