Can I use my own background image

Use an unsupported style tag in your guestbook "Layout".
You will need to check the "custom HTML" box to enter the following
style tag.

There must be some text before this style statement or it wont work.

<style>
BODY {background-image: URL(XXXXXXXXXXXXXXXXXXXXXXX);}
</style>


Replace the XXXXXXXXXXXXXXXXXXX with the address of the background image from your website beginning with https://

High contrast images for background are not recomended

Please do not use image addresses from someone else's web site.