I just want to add image to the header, when i add the following code the the header.ascx <img src ="images/banner.jpg" /> the image is then apear in the main page but when i navigate to any of the categories the images don't show and when i get the property for the image i found it : http://www.mywebsite.com/category/images/banner.jpg
You need to specify the image's location relative to the root of the site instead of relative to the current directory since categories and products use rewritten (virtual) URLs.
Change your image tag to the following: <img src="/images/banner.jpg" />
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I didn't see that this was already answered, the solution offered by jnet works as well.