I ran into this problem in SharePoint 2010 and discovered that the actual post from Live Writer works as intended, but the email that is generated by SharePoint incorrectly links the image.
Here is the real URL of where the image resides that was uploaded via Live Writer: ../Lists/Posts/Attachments/13/image_thumb_4C99B85E.png
Now, here is the URL that is referred to in the email alert: ../_attach/image_2_4C99B85E.png
Conversely, if a new post is created and a picture is uploaded via the built in Blog Tools,

the picture is uploaded by default to the Photos library of the blog site. Then, the email alert that is generated by SharePoint contains the
actual photo, not a link.
My work around is to use the built in blog editing tools if there is an alert on the posts list and I am posting something with a photo. If no photo is involved, or there are no alerts to worry about, Live Writer works fine.
MCSE + I