Snapshot version of Vivaldi 1.15 has added the ability to set the desired image as Vivaldi browser window background, this feature is still a work in progress, Vivaldi team promises to improve it more in future.
Following the addition of default search engine to Private Window, the 1.15 version of Snapshot has received yet another update. The update has added the possibility of having an image as Vivaldi window background and the capture setting set by the user to persist after the restart of the browser.
Vivaldi 1.15 : Set a background image for Vivaldi window
1. Click on V icon > Tools > Settings> Themes
2. You can find the new feature – ‘Background Window image’ – under Themes, click on ‘Select File’, choose an image and click Open,
3. After setting the background image option – the options available are: Scale to fit, Repeat and ‘Blend Accent Color from window with image’- see the browser window for the change.
Tip: Do you know Vivaldi allows to use custom background image for its start page? Read: How to Set a custom background image for Vivaldi browser Start Page?
Source: Vivaldi blog