I would probably remove the logo and text from that image and then make the grass a repeating image so that it goes all the way along the top of the website. Then just put the logo and text back on as separate elements.
If you adjust the grass images so that the left and right side form a seamless join then you can just repeat it on the body element at the top of the page.
Alternatively just add a fade to green at the edges of the image so that it merges into the background instead of stopping suddenly. Just create a green background that goes from 0 - 100% opacity and overlay it at each end of the image. I don't use photoshop so can't tell you how to actually do it in photoshop but in Fireworks its pretty straight forward.
For basic image manipulation, you should try Irfanview aka Iview. Nowhere near the possibilities of Photoshop, but extrememly user friendly. It's very stable, quick, simple and stangely enough, totally free.