changing image source in javascript not working in firefox

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HTML

I just want to change the image source through JavaScript. It works in IE but it didn't work in Firefox and Google Chrome.
Code is written below:

which_image_loaded = 0;
       numberofimages = 14;
       ImageNames = new Object();
       ImageNames.length = numberofimages - 1;
       for (counter = 0; counter < numberofimages; counter++) {
           file_number = counter + 1;
           filename = (".\\books\\\cred\\image" + file_number + ".jpg");
           ImageNames[counter] = filename;
       }


function imgthumb(imgnum) 
{
  document.getElementById("myimage").src = ImageNames[imgnum];
}