ximagesrc segfaults when trying to blend the X cursor over the frame if the bpp
of your X server is different than 32 (i've tested with 16). 
This: 
composite_pixel ((guchar *) & (((guint32 *) image->ximage->data)[j *
                      s->width + i]),
              (guchar *) & (((guint32 *) cursor_image->pixels)[(j -
                          cy) * cursor_image->width + (i - cx)]));
is correct only if every pixel is 32 bit wide. s/guint32/guint16 to make it
works with 16bpp. 
The function "composite_pixel" should also support different bpp and different
masks.

If anyone is faster than me i'll take a look ASAP.