I have a Dynamic map service running where I used the MXD document???s Frame>Background property to set the colour for the map background (i.e. the sea) to blue.
When I change the service type to cached, the ???sea??? background colour does render.
Thanks for your reply. I do include the transparent=false option when requesting the image - BUT
After some deeper investigation, it actually pans out that the problem came in with the hill shade image that I used as a backdrop: the image was not clipped at the sea borders, so it showed the image "white" overflow. After I set this right, the image exported correctly.
Sorry for this. At the end it was user error :rolleyes: