Thank you all, but I found out what is happening!
I'm using a large thumb on the left, which should show the thumb of the album.
It turns out the gallery on the right is counting on the presence of the thumb, and takes a picture.
Now, how can I fix this?
I did this thumb as follows:
<?php while (next_image(false, $firstPageImages)): ?>
<div class="thumb_image">
<h3><a href="<?php echo htmlspecialchars(getAlbumLinkURL());?>" title="<?php echo gettext('View album:'); ?> <?php echo getBareAlbumTitle();?>"><?php printAlbumTitle(); ?></a></h3>
<a href="<?php echo htmlspecialchars(getAlbumLinkURL());?>" title="<?php echo gettext('View album:'); ?> <?php getBareAlbumTitle();?>"><?php printCustomAlbumThumbImage(getBareAlbumTitle(), NULL, 233, 159,233, 159); ?></a>
</div>
<?php break; ?>
<?php endwhile; ?>
Is it correct? Is there another way to do this?