If you will be using short labels and images of the same height, you could set ImageAlign to Top, TextFormat.LineAlignment to Far, and set a node size that will fit the image and text.

Otherwise you can define ImageRectangle and text region for your shapes such that the ImageRectangle is at the top and the text region is at the bottom. E.g. adding this xml to the shape library file will allocate the top 3/4 for an image, and the bottom 1/4 for text:

<image x="0" y="0" width="100" height="75" ... />
<textarea>
  <line dash-style="Custom" width="-1">
<point x="5" y="75" />
<point x="95" y="75" />
  </line>
  <line dash-style="Custom" width="-1">
<point x="95" y="75" />
<point x="95" y="100" />
  </line>
  <line dash-style="Custom" width="-1">
<point x="95" y="100" />
<point x="5" y="100" />
  </line>
  <line dash-style="Custom" width="-1">
<point x="5" y="100" />
<point x="5" y="75" />
  </line>
</textarea> 

I hope that helps,
Stoyan