Format info (hat tip MikeS):

  /* Serialize to the appropriate format. The documention for WM/Picture says
     that it's compatible with id3's APIC tag. The documentation is wrong.

     The actual format is:
       uint8 picture_type;   // One byte for the picture type. This is
                             // compatible with the APIC tag's picture type,
                             // but in practice always has the value 0.
       uint32 data_length;   // Length of the actual image data
       uint16 mime_type[];   // Null-terminated, UTF-16-LE mime type.
       uint16 description[]; // Null-terminated, UTF-16-LE description, may be
                             // empty (just the null terminator).
       uint8 image_data[];   // The actual image data.
   */