Uploading images to use as the Background is no longer supported.
This change was introduced in nightTab 7.0.0
Using local images as the background is useful but also has many drawbacks.
As a rule, nightTab does not store any user data on a remote database. So any image files uploaded and used as a background has to be stored on the client side in local storage (in the browser cache). Local storage has a size limit (which varies between browsers) of about 5MB.
Images can very easily exceed 5MB in size. Which is a problem when the same storage is used to save user settings: the layout, bookmarks and theme settings. In this scenario the browser can throw errors or prevent nightTab from functioning as expected.
Removing several MBs of image data from the local storage also means nightTab will read saved data faster resulting in reduced load times.
Other issues with uploading images continue to appear. Some images just fail to load.
For the reasons above local image background upload controls have been dropped from the feature set.
To use an image as a background you should link to the image using a URL. Just paste one or more URLs into the Theme Background Image input to change the background image. This method is the most reliable and uses the least amount of storage space.