Change AJAX processing icon and progress bar in Drupal7 using CSS
You can do it by overriding the CSS in your theme CSS file. Here is the rule that you've to override

.ajax-progress .throbber {
  background: transparent url(../../misc/throbber.gif) no-repeat 0px -18px;
  float: left; /* LTR */
  height: 15px;
  margin: 2px;
  width: 15px;
}


You can just change the background and you will be able to change the processing icon.

Here is the another one that you should override

html.js input.form-autocomplete {
  background-image: url(../../misc/throbber.gif);
  background-position: 100% 2px; /* LTR */
  background-repeat: no-repeat;
}