Each YouTube video has 4 generated images, like:
http://img.youtube.com/vi//0.jpg
http://img.youtube.com/vi//1.jpg
http://img.youtube.com/vi//2.jpg
http://img.youtube.com/vi//3.jpg

The first one in the list is a full size image and others are thumbnail images. The default thumbnail image (ie. one of 1.jpg, 2.jpg, 3.jpg) is:
http://img.youtube.com/vi//default.jpg

For the high quality version of the thumbnail use a url similar to this:
http://img.youtube.com/vi//hqdefault.jpg

Alternatively, you can use the YouTube API to get thumbnail images.

I was looking a way to send email with attachments in drupal, to create a simple module which sends email to users with attached images.

I try to do it with drupal_mail() function. but results were no so great! so when finding alternative, I found a module called mimemail module which allow send
email attachments with less pain.

even though i sounds like pretty straight-forward task. I have dig little harder find the prefect combination.

header('Location: invoices/sample.pdf');

 $sender = 'mycompany@company.com';
 $recipient = 'myemail@email.com';
 $subject = 'New order';
 $body = 'Please, see the attachment.'; //body text in HTML format
 $plaintext = TRUE; //whether to send messages in plaintext-only
 $headers = array();
   $attachments[] = array(
    'filepath' => 'sites/default/images/sample.jpg' , //you have to use relative paths.
    'filename' => 'download.jpg' , // can use any alias name here
    'filemime' => 'image/jpeg' , // mime file type
 'list' => TRUE,
 );

mimemail($sender, $recipient, $subject, $body, $plaintext, $headers, $text = NULL, $attachments, $mailkey, $send= True);

I installed postfix and got this error:
postdrop: warning: unable to look up public/pickup: No such file or directory.

Turns out that sendmail was previously installed and that was messing things up. I had to stop sendmail and make the appropriate directory and restart postfix.

Specifically:

mkfifo /var/spool/postfix/public/pickup
ps aux | grep mail
kill
sudo /etc/init.d/postfix restart

Test emailing
#email youremail@abc.com (enter)
subject: test (enter)
test (ctrl+d)
cc: youremail@abc.com (enter)

if your receive above then email everything is fine now! :)