If you add the vertical align property to the input box you can align it with the button like this:
<input type="text" size="4" value="1" name="PracticeCart_1_Quantity_Add" style="vertical-align: top;"/>
You can also add the vertical-align: bottom to the button instead, but since there is a little bit of white space at the bottom of the image for the button, it doesn't line up as nicely.