i made a 3d view with background sky image and looks great on the cover sheet.
but when exporting to dwf it does not show up.
This is a known limitation for dwf export. As a workaroud, currently, you can try to create a sheet, and add the 3D view to that sheet, then insert a tif format image on that sheet, then export this sheet view to dwf. If this doesn't satisfy your requirement very well, I would encourage you report such wish list here:
You can use the form in the link to tell us what you think about our products, their features, and any improvements you would like to see. Then related people will read all the feedback recieved and take what our customers have to say very seriously.
would that be like place a sky image on tha plan sheet, them place a revit view on top of it and make sure the borders line up?
Yes. Maybe you still need to delete the view title to get a better result.