Can I replace the default image for System Pages?
The image used on the 404 and 500 Error Pages can be replaced in Theme Settings
Change image on 404 and 500 Error pages
From Theme Settings, click System Pages:
Expand the Error 404 settings and change the dropdown to Custom Image:
Use the file selector to Upload or Browse images and select the image you want to use.
If you don't want an image, there is also the option for No Image.
The same option is available for the Error 500 system page: