Yet another way to do it - it forces each row to consist of exactly 5 cells:

#usr/bin/perl
use strict;
use warnings;

my @Pics;  # test data
push @Pics, "/pics/pic_$_.jpg" for (1..13);

my @Cells;
push @Cells,
  qq|    <td><img src="$_"><br></td>\n|
  for (@Pics);

my @Rows;
while (@Cells) {
  my @Row;
  push @Row,
    shift(@Cells) || "    <td><br></td>\n"
    for (1..5);
  push @Rows,
    "  <tr>\n",@Row,"  </tr>\n";
}
print "<table>\n",@Rows,"</table>\n";

__END__
<table>
  <tr>
    <td><img src="/pics/pic_1.jpg"><br></td>
    <td><img src="/pics/pic_2.jpg"><br></td>
    <td><img src="/pics/pic_3.jpg"><br></td>
    <td><img src="/pics/pic_4.jpg"><br></td>
    <td><img src="/pics/pic_5.jpg"><br></td>
  </tr>
  <tr>
    <td><img src="/pics/pic_6.jpg"><br></td>
    <td><img src="/pics/pic_7.jpg"><br></td>
    <td><img src="/pics/pic_8.jpg"><br></td>
    <td><img src="/pics/pic_9.jpg"><br></td>
    <td><img src="/pics/pic_10.jpg"><br></td>
  </tr>
  <tr>
    <td><img src="/pics/pic_11.jpg"><br></td>
    <td><img src="/pics/pic_12.jpg"><br></td>
    <td><img src="/pics/pic_13.jpg"><br></td>
    <td><br></td>
    <td><br></td>
  </tr>
</table>
[download]

^~Django
"Why don't we ever challenge the spherical earth theory?"