There is another way too:
Change the attributes of thePostThumbnail in the template (index,page,..)
for the alt use array alt
thePostThumbnail(array('alt' => ''.get_the_title().''));
for the title use array title
thePostThumbnail(array('title' => ''.getTheTitle().''));
no need to change stuff in the functions.php
You can add lots of parameters, for example:
the_post_thumbnail( 'thumbnail', array('class' => 'alignleft post_thumbnail', 'style' => 'width: 186px;', 'alt' => 'no alt', 'title' => ''.get_the_title().'' ));
