$size = 200; // 200px
$positive = 20;
$negative = 20;
$total = $positive + $negative;
$positivePercent = ($positive / $total);
$negativePercent = ($negative / $total);
echo '<div style="width: '.$size.'px"><img src="positive.jpg" width="'.($size * $positivePercent).'" height="10" style="float: left;" /><img src="negative.jpg" width="'.($size * $negativePercent).'" height="10" style="float: left;" /></div>';