In your HTML part:
....
<img src="$BIGIMAGE" width="$BIGWIDTH" height="$BIGHEIGHT" border="0">
....
Then in PHP
$printstring = strstr("\$BIGIMAGE", $dbrow['imgname'], $printstring);
$size = getimagesize($dbrow['imgname']);
$printstring = strstr("\$BIGWIDTH", $size[0], $printstring);
$printstring = strstr("\$BIGIMAGE", $size[1], $printstring);
Then for each thumbnail link
<a href="thumbs.php?id=$IMGID">
Where $ID is the identifier in the database.
hth