Glad to hear you're working on creating some stylish Insights workbooks! While creating my own workbooks in the past I've found that a great way to achieve a page background is to add a Text/Media widget and upload my desired background image. From there I send it to the back (behind all other cards).
Using this method I'm able to maintain control over the exact extents of the image. Check out this example from a blog post where I added a galactic star field to my workbook page for a little extra visual interest.
