BUG: Correctly handle images with an alpha channel

Bug caught by Zachary Pincus on scipy-user.

scikits/image/io/_plugins/freeimage_plugin.py

@@ -320,10 +320,17 @@ def _array_from_bitmap(bitmap):
         return arr[..., ::-1].T
     if len(shape) == 3 and _FI.FreeImage_IsLittleEndian() and \
        dtype.type == numpy.uint8:
-        b = array[0]
-        g = array[1]
-        r = array[2]
-        return numpy.dstack( (n(r), n(g), n(b)) )
+        b = n(array[0])
+        g = n(array[1])
+        r = n(array[2])
+        if shape[0] == 3:
+            return numpy.dstack( (r, g, b) )
+        elif shape[0] == 4:
+            a = n(array[3])
+            return numpy.dstack( (r, g, b, a) )
+        else:
+            raise ValueError('Cannot handle images of shape %s' % shape)
+
     # We need to copy because array does *not* own its memory
     # after bitmap is freed.
     return n(array).copy()