Add the user's roles to the classes applied to a BuddyPress avatar.

add-roles-to-avatar-classes.php

<?php 
add_filter( 'bp_get_member_avatar', 'my_add_user_level_to_avatars', 10, 2 );
/**
 * @param string $value Formatted HTML <img> element, or raw avatar URL based on $html arg.
 * @param array  $r     Array of parsed arguments. See {@link bp_get_member_avatar()}.
 */
function my_add_user_level_to_avatars( $value, $r ) {
    global $members_template;

    // The args have already been parsed in bp_get_member_avatar. We'll use them except for the css class.
    $class = array( $r['class'] );
    // Get user roles.
    $user_info = get_userdata( $members_template->member->id );
    if ( ! empty( $user_info->roles ) ) {
        $class = array_merge( $class, $user_info->roles );
    }

    // Convert array to space-separated string.
    $class = implode( ' ', $class );

    return bp_core_fetch_avatar( array( 'item_id' => $members_template->member->id, 'type' => $r['type'], 'alt' => $r['alt'], 'css_id' => $r['id'], 'class' => $class, 'width' => $r['width'], 'height' => $r['height'], 'email' => $members_template->member->user_email ) );
}