In the HTML Editor for your email design:
  1. Select the "Insert" tab
  2. Click the "Table" button
  3. In the Insert/Edit Table window, select the "Advanced" tab
  4. Click the Browse button next to the field labeled "Background Image"
  5. Select the desired background image from the Image Library and then click the "Insert" button
  6. In the Insert/Edit Table window, click the "Insert" button
NOTE: You may also insert background images to a single Row or Cell within the Table by selecting the relevant Row or Cell and clicking the "Row properties" or "Cell properties" buttons on the Table tab of the HTML Editor.  The background image selector is on the "Advanced" tab of the Row/Cell properties window
 
Please see the Blackbaud NetCommunity Program Basics user guide for more instructions regarding use of the HTML Editor and adding/editing tables therein.