How to add CSS classes to a an image which belongs to a content type?
You can use hook_node_preprocess in your themes template.php file to add to the 'attributes' array for the node before the variables are sent to your node template. when your field gets built up into html it will then have your class.

If you enable the devel module you can add the following to template.php to see what this 'build array' which is used to render the node looks like.

function twt_preprocess_node(&amp;$vars) {

    dpm($vars);

}


To alter the classes added to your field use something like the following (though look to the above to find out if you are putting your class in the right place).

function twt_preprocess_node(&amp;$vars) {

    $vars['field_myimagefield']['#attributes']['class'][] = 'myclass';

}


Some useful reading:


Render Arrays in Drupal 7
Setting up variables for use in a template
hook_preprocess_HOOK (Drupal 7)