Hiya! It's fairly easy. What you have to use is a control-structure (don't let me fool you, statements are just made of commands like IF, ELSE and such)
Adopt this example and see what you get out of it :
<?php
if ($image) {
echo "<td width='118'><img src='images/$image'><br></td>\n";
} else {
echo "<td width='118'><img src='images/default.gif' width='15' height='1'><br></td>\n";
}
?>
Note that I've stripped the height and width-attribs when you're going to show a picture. Or else it would have become VERY low (height=1 ?)
Hint : Go have a look at IF, ELSE and other commands at http://www.php.net/manual/en/control-structures.php