Divide your image in 3 pieces, the left blue part (leftblue.gif), the right blue part (rightblue.gif) and just a 1 pixel gray middle (dialog_bg_n.gif).
When the table starts, it starts with a line ( <tr> ), and inside that line it has cells ( <td> ), now if you see your dialog.tpl it only has one cell, so divide it into 3 cells like this:
Code:
<td>[img]{$ImagesDir}/leftblue.gif[/img]<td>
<TD height=15 class=DialogTitle background="{$ImagesDir}/dialog_bg_n.gif" valign=bottom>{$title}</TD>
<td>[img]{$ImagesDir}/rightblue.gif[/img]<td>