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There are a couple of ways you might approach this. And I'm not sure if one would be any better than the other.
1 - Path menu > Trace Bitmap. This will convert the image to vector. But since you need such a simplified image for the t-shirt, you'll probably have to do some node editing, to remove a lot of the odd bits and pieces which will inevitabley result from doing the automated trace. I would suggest trying the SIOX Foreground Selection technique:
http://tavmjong.free.fr/INKSCAPE/MANUAL ... -SIOX.html. It might not work, but if you get the settings right, it might be perfect.
2 - Trace it by hand using the Pen or Pencil tool. You would Import the photo, and then just draw right on top of it, around the hair and face, to create a vector path.
I suspect the automated trace might be a little easier, but I guess it depends on your Inkscape skills.