Strange behaviour with images in sample CSS files

[roadtorode]

I have been learning the product using a simple site and the templates/CSS supplied with CMS 1.8

I have used a particular JPG file from upload/images happily in the template 'Standard'.  When I moved over to template "Top simple navigation + left subnavigation + 1 column", the header image is in the layout css file.  However, when I substitute the 'known good' jpg in here, it does not display.  Using chrome's 'inspect element' I note this file fails with a 404 and also causes the error "resource interpreted as image but transferred with MIME type text/html" - some other random images from the same directory and with the same permissions DO display in this location.  I'm stuck......

[Jeff]

Please post your css and your template and provide a link if possible.