load src of images that attachs in wordpress’s post

The following line is incorrect as showposts is deprecated.

$recent = new WP_Query("showposts=30");

Instead use,

$recent = new WP_Query('posts_per_page=30');

Following that,

Does the thumbnail size of 80×80 exist?

array(80,80)

If not try any of,

$thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), small );
$thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), medium );
$thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), large );

Remember each of small, medium or large – refers to different filenames or src – so if you want to retrieve the full resolution image, original upload then use large.

If you must return the src URL as

http://domain.com/image_80x_80x.jpg //example

…meaning that you definitely want the 80×80 version and it does not exist then you will need to regenerate your thumbnails.

You can use: http://wordpress.org/extend/plugins/regenerate-thumbnails/

If you did not add the image size of 80×80 to your theme you can do so using the add_image_size function in your functions.php file,

add_image_size( 'my_custom_size', 80, 80 );

Now, when uploading new images, you will also get a 80×80 version. You can then refer to it as mentioned above by either,

wp_get_attachment_image_src( $id, array(80,80) ); //via array specifying pixel w & h

or

wp_get_attachment_image_src( $id, my_custom_size ); //via name used in add_image_size