get_avater $default

<?php echo get_avatar($id_or_email, 50, $default_avatar ); ?>

Three questions:

1. According to the codex, to change the default avatar I would use $default = ‘<path_to_url>’ as the last parameter. How do I make a relative call to an image inside <?php ?>. I’d like to do it similarly to how I call images –

<img src="<?php bloginfo('stylesheet_directory'); ?>/img/someimage.jpg" alt="alt text here" />

2. How can I add alt text using the get_avatar function?

3. How can I link the gravatar image?

Thanks for any help you can provide.

Thank you,

Joe