The auto image resizing is only applied to local images inserted through WP in "full size" mode as it was causing issues with external images in earlier Atahualpa versions.
There is however a jQuery script in the header that resizes all "images inside posts" for IE6. You could make that work for all browsers, and let me know if that works well.
This would not re-resize images when the visitor resizes his browser window. But that shouldn't happen very often.
To do this, find in header.php
if ($bfa.browser.msie && /MSIE 6\.0/i.test(window.navigator.userAgent) && !/MSIE 7\.0/i.test(window.navigator.userAgent)) {
and delete it, and about 28 lines farther down, find
});
}
and change it to
});