Theme user pictures in comments in drupal 7
To help you understand the syntax, have a look at the comment.tpl.php documentation, here you will find the different variables and a link to template_preprocess_comment() where they are defined:

$variables['picture']   = theme_get_setting('toggle_comment_user_picture') ? 
                          theme('user_picture', array('account' =&gt; $comment)) : '';


So there's a theme function to render this picture: theme_user_picture() which state you should have a look a the template file: user-picture.tpl.php which finally send you to template_preprocess_user_picture().

There you can override this function copying and pasting the original code and changing the bits and parts you want to suit your needs (put this code in your template.php file):

function YOURTHEME_preprocess_user_picture(&amp;$variables) {
  //change only the relevant part and leave the rest of the code
  //For Drupal 7 imagecache is in core so I put the correct syntax
  $variables['user_picture'] = theme('image_style', array('style_name' =&gt; 'size32x32', 'path' =&gt; $filepath, 'alt' =&gt; $alt, 'title' =&gt; $alt));

  //...
}


Have a look at the $variables to find a way to target comments or add a class there and use CSS as dkiscool mentionned.

NB: if you don't like browsing all the API documentation, there's a faster way, use theme developer (see this answer).