Why Type="image" Can't Work Under IE

Code:
$html .= '<div id="submit"><input type="image" name="subscribe" onmouseover="this.src='images/1_hv.jpg'" onmouseout="this.src='images/1.jpg'" src="images/1.jpg" value="'.$data["button"].'" onClick="return checkform();"></div> ';

why the type="image" code can't work under IE? when i click the image under IE, it can't work. it can't submit the form. but under firefox and chrome.it can work. how to correct it ? when i change the input into

Code:
<input type="submit" name="subscribe" value="'.$data["button"].'" onClick="return checkform();">

when under IE,firefox,chrome. all can work