the image being used for the background is found here:
like Ian has already stated, this image is set to a width of 150px, to be able to use this image as a gradient background for your design, it will need to be edited to be 175px wide to match the menu width you are trying to create.
In the menu writer wizard, you can click the Edit design button to make design changes, if you want to use a custom image file, go to the design tab, then click the hover tab you can select an image to use for the a tag, the span tag, or the LI tag.
on the CSS menu writer support page, in the archived documentation section, there is a tutorial named "Changing the Design to use custom button images" that you may find helpful, it was created for menu Writer 1 but the concepts are still applicable.
Ian S is not a WebAssist employee, but he is a very helpful member of this forum.