I need a code to download images not to view them

First, setup your file field so that it saves into a separate directory. This will make it so that you will only force certain files to download and not affect the rest of the files on your Drupal site. Visit Administer -> Content management -> Content types (admin/content/types). Click "Manage fields" on the type that you want files to be forced to be downloaded. Add or edit a file field, under the "Path settings" fieldset, set a path for your files to be uploaded.
Now files uploaded to that field will be saved in a separate folder under your sites files directory.

FTP to your site and create a file named .htaccess in your files directory. Usually this will be sites/default/files/[name of the subdirectory from step 1]. In that file put the following lines:
<FilesMatch "\.(?i:doc|odf|pdf|rtf|txt)$">
Header set Content-Disposition attachment
</FilesMatch>
This particular example will force files with doc, odf, pdf, rtf, or txt to download instead of being shown in the browser.

Upload a few files and (using the "Generic files" display setting), the files will be downloaded when the user clicks on the link.
You may need to load mod_headers in Apache2 for this to work.
Assuming you have root access it can be done as follows:
# a2enmod headers
# /etc/init.d/apache2 reload