To use an image as a form background, edit the Style that is used for the DataPage. On the Define Style Settings screen, click on the Source tab to see the CSS codes. In the object cbFormtable, add this:

.cbFormtable {
background-image:url('http://enter_complete_url_of_the_image');
}

Then comment out the background color styles in the following classes:

.cbFormTableRow
.cbFormTableEvenRow
.cbFormTableRow_hover
.cbFormTableEvenRow_hover