Mail signature logo shows as attachment on Outlook

the message is being sent as text format and not rich text or HTML.
on the Mail App, check the default format of mail during the send and also confirm if you are able to receive HTML/RTF mails on exchange from other clients/domains.

are you able to receive mails to your exch acct from other domains like yahoo or hotmail in RTF?

Outlook is very bad at handling anything but Outlook.
There various workarounds to get the email formatted so that Outlook doesn't choke.

First, set the attachment prefs to always send windows friendly and put all attachments at end of message.
Second, you will need to change the message font to something other than your default font. One way to make this easier is to create an automatic signature that uses the font you want in the first line. When you start a new message, click in the first line of the signature and go from there. Make sure default font is not same as that signature font.

Or, just send plain text.

Message was edited by: Barney-15E

I had the same problem it fixed it as follows.
1)Send the image that you want to use to some one that has outlook on a pc.
2)Ask them to place this image in their signature in outlook. (This will create the right formatting and code within the email for the signature to work(according to outlook))
3) Ask them now to send you an email with this signature attached ( the modified one with your signature image)
4) Open the email from your friend that sent you back your signature image. Wipe over the image to select it then copy.
5)Open Preferences in mac mail. go to signature section were you want to place the image
6) Past into the mac mail signature pain.

Job Done.
Woo Hooo!