Applying image style to Privatemsg
This is off the top of my head with some parts but I think this is the way you would do it.

print theme('image_style', array('path' =&gt; $message-&gt;author-&gt;picture-&gt;uri, 'style_name' =&gt; '{image-style}'));

With {image-style} replaced with image style that you want like thumbnail.

If this doesn't work I will take a second look.