showing alt text instead of iamge

i have in my theme a problem its showing a large image in portfolio page and portfolio slider, now the slider get the image small (cropped) which is very bad looking, and i would like it show another image (smaller one i choose) taking the url from the alt tag in the big image.

my code currently is:

<div class="portfolio-thumb">
                        <?php
                        $image = $portfolio->get_image( 'portfolio_section', array( 'class' => 'img-responsive' ) );
                        if ( strcmp( $image, '' ) != 0 ) {
                            echo $image;
                        }
                        else {
                            ?>
                            <img src="<?php echo $portfolio->get( 'baseurl' ) ?>images/no-image.jpg" class="img-responsive" />
                        <?php
                        }
                        ?>

i tried modifying the

$image = $portfolio->get_image( 'portfolio_section', array( 'class' => 'img-responsive' ) );
                        if ( strcmp( $image, '' ) != 0 ) {

to be

$image = $portfolio->alt_image( 'portfolio_section', array( 'class' => 'img-responsive' ) );
                        if ( strcmp( $image, '' ) != 0 ) {

but i think this is wrong as when i apply it the full front page dont work…

any one could help?
thanks
mark