@Seth Battin, @ Ayesh K, thanks for the tips,
working code...
global $user;
$newnode = new stdClass();
$newnode->title = 'New node title';
$newnode->body = "this is a new node, created by import function";
$newnode->uid = $user->uid;
$newnode->type = 'theme_preview';
$newnode->status = 1;
$newnode->promote = 0;
$newnode->active = 1;
$newnode->field_theme_preview_demo_link[0]['value'] = 'test 1';
$newnode->field_theme_preview_content_styl[0]['value'] = 'Books';
$field = field_file_save_file(
'sites/default/files/srcImage1.jpg',
array(),
'sites/default/files/theme_preview_thumbs');
if( !isset($field['data']) ) $field['data'] = array();
$field['data']['title'] = "the image title";
$field['data']['alt'] = "the image alt";
$newnode->field_theme_preview_thumb = array(0 => $field);
$newnode = node_submit($newnode);
node_save($newnode);