Here is the modified working code...
<section id="content">
<?php
$i=1;
while(have_posts() && i < 7):the_post();
$tumbUrl = '';
if(has_post_thumbnail())
{
$tumbUrl = wp_get_attachment_url( get_post_thumbnail_id($post->ID) );
}
if(i < 4):
?>
<div id="row1">
<div id="tile<?php echo $i; ?>" class="tile">
<div id="img<?php echo $i; ?>" class="tileimage"<?php if($tumbUrl != ''): ?> style="background-image:<?php echo $tumbUrl; ?>"<?php endif; ?>></div>
<div id="text<?php echo $i; ?>" class="tiletext"><a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_title(); ?></a></div>
</div>
</div>
<?php
else:
?>
<div id="row2">
<div id="tile<?php echo $i; ?>" class="tile">
<div id="img<?php echo $i; ?>" class="tileimage"<?php if($tumbUrl != ''): ?> style="background-image:<?php echo $tumbUrl; ?>"<?php endif; ?>></div>
<div id="text<?php echo $i; ?>" class="tiletext"><a href="<?php the_permalink(); ?>" rel="bookmark" title="a<?php the_title(); ?>"><?php the_title(); ?></a></div>
</div>
</div>
<?php
endif;
endwhile;
?>
</section>
Note: Make sure the ending mark(;) are there and also the space as required.