Fix errors in image processing tutorials

[aa-dit-yuh]

Bug report:

- ./_site/tutorial/imageprocessing/enhance/index.html
  *  internal image /img/tutorial/img_processing/enhance/pic5.png does not exist (line 1)
- ./_site/tutorial/imageprocessing/gscale_bin_hist/index.html
  *  External link http://opencv.willowgarage.com/documentation/histograms.html failed: response code 0 means something's wrong.
             It's possible libcurl couldn't connect to the server or perhaps the request timed out.
             Sometimes, making too many requests at once also breaks things.
             Either way, the return message (if any) from the server is: Couldn't resolve host name
- ./_site/tutorial/imageprocessing/installopencv/index.html
  *  internal image /img/tutorial/img_processing/ins_opencv/pic4.JPG does not exist (line 1)
- ./_site/tutorial/imageprocessing/noise_reduction/index.html
  *  internal image /img/tutorial/img_processing/noise/pic1.gif does not exist (line 1)
  *  internal image /img/tutorial/img_processing/noise/pic1.gif does not exist (line 1)
- ./_site/tutorial/imageprocessing/roi/index.html
  *  internal image /img/tutorial/img_processing/roi/pic6.png does not exist (line 1)