You have a blank trailing row if the number of images is divisible by 5. Fix:

$i = 0;
$open = 0;

foreach (@images)
# or: while ($sth->fetch)
# or: while (readdir(IMAGE_DIR))
{
   (print "<tr>", $open = 1) unless $open;
   print "<td>";
   # print image
   print "</td>"
   (print "</tr>", $open = 0) unless ++$i % 5;
}

print "<tr>" if $open;
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As a bonus, I wrote the fix such that you don't have to know how many images there are in advance (just like in the original solution). This is very useful when reading from a database.