If you are using a User ID field, this function is not needed. Use this function if you are collecting email addresses from users who are not logged in, and would like to show an avatar using only the email address. [25 avatar=40] 25 = the field ID 40 = the size of the avatar

add_filter( 'frm_display_value', 'frm_add_avatar_shortcode_option', 10, 3 );
function frm_add_avatar_shortcode_option( $value, $field, $atts ){
	if ( isset( $atts['avatar'] ) ) {
		$value = get_avatar( $value, $atts['avatar'] );
	}
	return $value;
}