Image streamer has been modified

Few things that I realize during mavconn usage

> image streamer
  - Linux qgroundcontrol does not display the streamed image
  - I've modified as in this commit

> Mavlink lookup
 - I've installed mavconn, mavlink in same root folder (let say, ~/src)
 - I built mavlink by 
    $ mkdir build
    $ cd build
    $ cmake ..; make
 - However, the folder used by FindMAVLINK.cmake sees something different folder.
 - I'm not sure this will always be a problem or not.