Q:

Change AJAX processing icon and progress bar in Drupal7 using CSS

Can I change the throbber/ajax loading icon and progress bar in Drupal 7 by adding new pictures and overriding CSS system files?

A:

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;
}

