I want to change the ajax gif loader for Drupal (6) without modifying the gif itself and without manipulating the main css files ... but redefining it from my css theme.
What is the best way to achieve this?
If you mean the loader misc/throbber.gif, you can just override it in your stylesheets using classes html.js input.form-autocompleteand .ahah-progress .throbbercss
misc/throbber.gif
html.js input.form-autocomplete
.ahah-progress .throbber