First adjust the width and height to the size that you need in your functions.php file –
add_image_size( 'home-portfolio-thumbnail', 150, 150, TRUE );
Then download and run a plugin called Regenerate Thumbnails – http://wordpress.org/extend/plugins/regenerate-thumbnails/. This plugin will automatically regenerate all of the image thumbnails of your site to adjust to the changes you made in your functions file.
Let me know if this works.