I've tried the 'align', 'imagealign', and 'valign' properties (in this case with the value of "top") in my style definition, my menu definition, and in menu items, all with no effect -- my images are being vertically aligned to the middle. See http://www.capitalandanimalhospital.com ... /test.html
I have never tried it myself, but according to the menu item properties quick ref:
imageposition (30) Will change the position of the associated menu item image. Valid values are top or bottom.
The imageposition property can also be used as a style property.
An alternative might be to do it the old-fashioned way... if the image is a gif, you could effectively shift it up or down by adding transparent space to the bottom or top of the image, without adding much to the size of the file.