When doing this, you'll want to edit your image in photoshop so that it perfectly fits the dimensions of your table. In this example, the image is at File:Odranoelluta image border.png and is 512 × 530 pixels. But the table itself is NOT 512 × 530 pixels, so things look all screwy.
Note: You can't set the background image with inline CSS. You'll need to be an admin on your wiki so that you can edit MediaWiki:Common.css to apply the necessary CSS rules.
See how the image is repeating itself? That kind of thing can happen if the image doesn't fit the size of the table. But you can use background-repeat: no-repeat; to disable the repeating of the background image. More info at http://w3schools.com/css/css_background.asp