Hi Lisa,
I think in this case, as I don't have Powerstore 4 (I have PowerStore 3) and I don't know how changing style for this element in the stylesheet will impact on other areas, to be absolutely safe I would add an in-line style to the <a> link directly in products_results.php.
In Powerstore 3 it is at about line 462:
<div class="imageWrapper"> <a href="Products_Detail.php?ProductID=<?php echo(rawurlencode($row_WADAProducts['ProductID'])); ?>" title="<?php echo $row_WADAProducts["ProductName"]; ?>"> <img class="WADAResultThumb" border="0" src="imagesupload/<?php echo $row_WADAProducts['ProductThumb']; ?>" /></a> </div>
just add the style: background-color:#FFFFFF; before the end of the opening <a> tag:
<div class="imageWrapper"> <a href="Products_Detail.php?ProductID=<?php echo(rawurlencode($row_WADAProducts['ProductID'])); ?>" title="<?php echo $row_WADAProducts["ProductName"]; ?>" style="background-color:#FFFFFF;"> <img class="WADAResultThumb" border="0" src="imagesupload/<?php echo $row_WADAProducts['ProductThumb']; ?>" /></a> </div>
That means that it will apply to all products but just in that link, and to no other elaments in the site. You could give that a try.