I see several options here:
Change save
Save your image to 20x20, and screams, the problem has disappeared.
Use CSS3
CSS3 introduces new backgroud size feature
Just put this in your CSS:
background-size: 20px 20px;
But it will not be compatible with older browsers.
Use JavaScript [Bad Style]
HTML
<form method="POST" action="test.php" name="myform"> <a onclick="javascript:document.forms['myform'].submit ();" ><img src="myimage.png" onclick="submit();" /></a> </form>
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