You need to do a while loop to pull each rowset. mysql_fetch_array() will only pull one row at a time.
Consider this solution:
<?php
$dbhost = 'xxxx';
$dbuser = 'xxxx';
$dbpass = 'xxxx';
$dbname = 'xxxx';
$conn = mysql_connect($dbhost, $dbuser, $dbpass) or die ('Error connecting to mysql');
mysql_select_db($dbname, $conn);
$result = mysql_query("SELECT * FROM mytable", $conn);
<table>
while ($row = mysql_fetch_array($result)) {
echo '<tr align="center"><td>' . htmlspecialchars($row['Picturedata']) . '</td></tr>';
}
?>
</table>
