i'm using 800x600 before, but when i change my settings to 1024x768 the background image turned smaller & does't fit the page. So how can i set it to fit the page even if it is 800x600 or 1024x768? does it have auto setting to fit the page whatever pixels i use?
Unfortunately not, no. CSS
Unfortunately not, no.
CSS at the moment doesn't have a way of resizing background images. IIRC, CSS3 will implement it, but at the moment no.
There are a few common ways designers deal with screen resolutions:
Create a simple background, eg a gradient, that only needs to fill up part of the screen.
Use a single fixed image in the corner of the screen (see www.roffles.co.uk)
Create a massive background image, one that will display more or less of itself depending on the screen resolution.
Alternatively, you can resize your image in a graphics editing program such as the GImp or Paint Shop Pro - however, then it will be too large for users with lower resolutions.
Can you show us the image? Then we can advise on the best way to do it.