<table class="header">
<a href="index.php">
<img src=""src="http://сайт.ру/forum/images/logos/day.jpg">
</a>
</table>
<table class="header">
<a href="index.php">
<img src=""src="http://сайт.ру/forum/images/logos/night.jpg">
</a>
</table>
header a > img[src$="day.jpg"] {...}
header a > img[src$="night.jpg"] {...}