upload of image with CKEditor plus KCFinder resulting in page not found

I've just learned about the existence of such plug-in. I'm trying to implement an image upload feature using KCFinder. I've also used the standard version of CKEditor, that contains a button for image uploading, without a "Upload" tab.

Instructions tell me to add some lines in CKEditor's config.js, indicating the KCFinder's browser and upload scripts, as follows:

config.filebrowserImageBrowseUrl = '/kcfinder/browse.php?opener=ckeditor&type=images';
config.filebrowserImageUploadUrl = '/kcfinder/upload.php?opener=ckeditor&type=images';

I've tried many different ways to specify the location of these scripts, but no matter what I do, when attempting to upload the image to the server, an error message will show up:

Sorry, the page you are looking for could not be found.

Tons of tutorials, but none help me.

via Chebli Mohamed