Change value of a button in a form
Yes, you can done it in form_alter . first need to know what is your button name in $form in form_alter, I suggest you print_r or var_dump $form and found your button (buttons usually are in #actions) . after it I suggest you handle your job with add a css class or id to your button and with css style it .

function yourcustom_form_alter(&amp;$form,$form_state) {
  $form['actions]['yourbuttonname']['#value'] ='';
   $form['actions]['yourbuttonname']['#attributes'] =array('class'=&gt;array('yourcssclass'));
}