Converting and save images as JPG in IMCE module during uploading?
It can be done in this way:


download ImageCache Actions module and enable Color Actions    module
create image style "Change file format Convert to: jpg"
download and install IMCE Image Style module
on the IMCE Image Style administration page select necessary style


And we need to add custom submit function

/**
 * Implements hook_form_FORM_ID_alter().
 */
 function mymodule_form_imce_upload_form_alter(&amp;$form, &amp;$form_state, $form_id) {
   array_unshift($form['fset_upload']['upload']['#submit'], "mymodule_upload_form_submit");
 }

/**
 * Change uploaded image extension.
 */
 function mymodule_upload_form_submit($form, &amp;$form_state) {
   if ($_FILES['files']['type']['imce'] == 'image/png' &amp;&amp;    $form_state['values']['imce_image_style']['optimize_image'] != '0') {
      $_FILES['files']['name']['imce'] = str_replace('.png', '.jpg', $_FILES['files']['name']['imce']);
   }
 }