How to set alias url for image address in a content type? I added an image field for a content type and its directory is /content_type_image. it works correctly but when I add a content to this content type the uploaded image url on the website is www.mysite.com/sites/all/ .... I mean it is clear where this image is in the server directory. Is there anyway to set an alias url for files and images in order to hide such urls?
You could set your fields to use the private file system. For more information see Working with files in Drupal 7.

With the private file system, the path to files is of the form

www.example.com/system/files/path/to/file.png

The path /system/files is a drupal url, not an actual directory on your file system.

For your drupal file setting you set the path of the private file system to something like /sites/default/files/private so your images are on the server at sites/default/files/private/content_type_image

So the path to the image will be

www.example.com/system/files/content_type_image/path/to/file.png

then when a user hits that url the drupal callback for the system/files will check access control and then load the image from

/sites/default/files/private/content_type_image/path/to/file.png

Using the private file system also means if someone doesn't have access to the content the image is attached to then they cannot browse directly to that file if they can work out the path (and google can't index it) and it means that if you are using revisioning, users cannot see files attached to old revisions of your content, whcih they can with the public file system.

This is probably the best solution you'll get for stopping users from knowing the location of your files and also from accessing files they shouldn't be allowed to.