Value cannot be null. Parameter name: source

Hi All - I'm really stuck and need your help.

Background:  The error occurs on a photo gallery image approval page and only occurs on approximately 10% of images submitted into the photo gallery.

I'm getting the following error:
Value cannot be null. Parameter name: source

on the PropertyItem line shown below:
Bitmap MyPhoto = new Bitmap(Server.MapPath(".") + "/tmp/" + strFile);
PropertyItem[] props = MyPhoto.PropertyItems;    

Does anyone know either a fix or a work around?  I'm kind of desperate here!

Cheers all,
Rob
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