With your Web page open, select File » Properties or right-click the page and select Page Properties. Then click the Formatting tab. Turn on the Background Picture checkbox and browse to find and select the image. If you want the image to remain fixed in the background as a viewer scrolls down your page, turn on the "Make it a watermark" checkbox.
If your background picture creates a tile effect, appearing multiple times within the page, then it's too small. Edit the image to make it large enough so that only one picture appears in a browser window. The problem is your visitors will all have different size windows. To keep the picture from tiling, you can give it a "no repeat" style. To do so, Select File » Properties and click the Advanced tab. Then click Body Style. Within the Modify Style dialog box that opens, click Format and select Border. Within the Borders and Shading dialog box, click the Shading tab. Under Background Picture, browse to and select the image. Then within the Repeat drop-down list, select No Repeat. Preview this effect in browsers with varying screen dimensions (Testing With Different Browsers).