Background with image is not besing saved

I use image as background for screenshot and it is not being saved properly. I see below error in console

```
app.js:1550 Error saving state: DataCloneError: Failed to execute 'put' on 'IDBObjectStore': HTMLImageElement object could not be cloned.
    at saveState (app.js:1548:15)
    at updateCanvas (app.js:6742:5)
    at img.onload (app.js:4215:21)
```

Thanks for creating such a great tool!