Why not try altering your SQL Statement to use this
syntax:
SELECT id, name, ISNULL(pictureurl, 'Default.jpg') as PictureURL
FROM Users
This will replace any null values in field "pictureurl" with "default.jpg"
or
try this:
<img>
src='<%#iif(container.dataitem("image")="","images/no_img.gif",container.dat
aitem("image")) %>'></img>
hope this helps,