There is a reason this works sometimes, and sometimes not.
I have done some testing on this with an IT colleague running Outlook. It appears that sometimes Mail on the Mac sends the message as a text e-mail with an attached JPG and sometimes it sends it as an HTML message with the image imbedded. If I send a message with the signature set to none, and just one image attached using the attach button (not drag and drop) in the toolbar, it comes across as a separate attachment. As long as I do not enter any text in the message box. If I type something in the message, or even tab into the text entry box so that a cursor appears there, it sends the message as HTML. If I add multiple images, it sends it as HTML.
You can check how your message was sent by looking at your sent messages folder, select the questionable e-mail message and hit Option, Command, U to view the raw source of the message. If it says "Content-Type: text/html;" anywhere in there, you will have imbedded images. If it says "Content-Type: image/jpg;" you can be sure that the image was received as an attachment.
With previous versions of Mail I was able to send multiple images that came through as proper attachments, showing the "Content-Type: multipart/mixed;" but I am not currently able to replicate that.
So, the only surefire way I have found to send a JPG as an attachment to someone using Outlook on Windows is to send one JPG at a time, with no signature and no text in the body of the message. Or, if you have multiple images, archive them in the Finder, and attach the archive.
...of course, as always, YMMV!