How can I programmatically validate that an user has uploaded an image in CCK?
You may have to use hook_form_FORM_ID_alter().Define your own module and in your module try the following code

function yourModuleName_FORM_ID_alter(&amp;$form, &amp;$form_state) {
  $form['#validate'][] = 'yourModuleName_FORM_ID_validate';
}

//your validation function
function yourModuleName_FORM_ID_validate($form, &amp;$form_state) {
  if (isset($_FILES['files']) &amp;&amp; is_uploaded_file($_FILES['files']['tmp_name']['machine_name_of_the_cck_field'])) {
    // File uploaded
  }
  else {
    //File not uploaded
  }    
}