I’ve had this problem for a while, that Mac OS X Mail (the native mail client in OS X) sends emails that other email clients don’t recognize as attachments, but as embedded pictures. This has been really frustrating, because if I’m sending an image to a client that needs to be saved on the client’s computer, he or she is unable to do so because the image is not a “real attachment”.
So, here are the facts: Wether an image is embedded in the email or not, it’s still an attachment. Thus, OS X Mail does nothing wrong when it embeds the image, other clients simply doesn’t want to understand that it’s still an attachment. The clients (read: “Outlook”) offer no way to save the image other than to right click it inside the email, but that only generates a .bmp that’s scaled down to the email size in pixel width.
Please note that it doesn’t matter if you click the Attachment button in the main taskbar or simply drag the images to the email body.
The solution:
The reason my images were getting embedded was because I was using some signatures with colors in them. As soon as you set the font, a color or similar, Mail turns the email into a “Rich Text” email, which basically means it’s a HTML email. So when you attach images to such an email, Mail generates an tag for that image, thus embedding it into the email itself. You can avoid this easily by just using Plain Text emails (see screenshot).
You can also try to simply don’t use any formatting in the email, including your signature.
Hope this helps. 
January 7th, 2009 at 7:34 am
Thank you. Thank you. Thank you.
January 7th, 2009 at 8:06 am
You're welcome!
February 9th, 2009 at 8:46 am
THANKS! SO SIMPLE BUT SO DIFFICULT
February 9th, 2009 at 11:28 pm
I know. Stupid Mail
February 18th, 2009 at 3:11 pm
Its taken me 7 months to get this right! My whole office is happy too! especially client service who uses pc
February 25th, 2009 at 8:48 pm
you're the man, great solution!!!
March 3rd, 2009 at 3:16 pm
Thank you so much, very informative!
March 3rd, 2009 at 6:24 pm
Here another problem, my sign with this configuration appear as attachment
March 24th, 2009 at 1:58 pm
Oh Thank God.. You da Man!
March 29th, 2009 at 11:10 pm
test
July 23rd, 2009 at 4:21 pm
Sometimes it’s really that simple, isn’t it? I feel a little stupid for not thinking of this myself/earlier, though.
July 23rd, 2009 at 10:38 pm
Yeah, it's that simple
December 3rd, 2009 at 11:07 am
So helpful, Thank you
March 5th, 2010 at 5:24 pm
Ok, but clearly Microsoft has solved the problem of being able to have rich text and html emails that ALSO allow attachments to deliver properly, whereas MAC has NOT solved this problem effectively. I love MAC, but why haven't they set this up properly? I hate to see Microsoft solve something that MAC hasn't!
March 5th, 2010 at 5:48 pm
I'm 100% with you.