Image tag will work at 100%, but only if the image dimensions are larger than the 100%. Outlook doesn't stretch past the default image width, which in your case is only 1px.
VML should also work, try emailbg.net
Question
I'm trying to display an image next to my 600px Content-Table on the left and right. The image is 1px wide and 400px high and should be streched or tiled on the X-Axis.
Ofcourse it works with different approaches in all clients but Outlook 07/10/13.
My first approach was a background-image using various VML-Methods like v:rect, v:background or v:image which all did not work.
My latest approach is to simply use an img-Tag and use width:100%, width="100%" and even mso-width-percent:1000.
Sadly, none of the 100% methods seem to work in any Outlook later than 03.
La solution
Image tag will work at 100%, but only if the image dimensions are larger than the 100%. Outlook doesn't stretch past the default image width, which in your case is only 1px.
VML should also work, try emailbg.net