In Tiam you can configure the main aspects for the 404 error page like: add an image, edit the title, the subtitle and the text.
To do this go to your WordPress dashboard and then click in appearance->customize and then click on Tiam: General Settings link.
Now Click on Page 404 link.
Here you can edit all fields you see.
When you’ve finished with the editing you can save your changes by clicking on Publish button on the top left corner of the page.