Last tested: Sep 2109
To send or download data in image-based formats, such as PNG or PDF files, Looker requires a rendering software. So, the option only appears if you have the rendering software installed.
The rendering software that is currently used is
Chromium . This is automatically installed for Looker-hosted customers. If you have a self-hosted instance, you can follow this docs page to set it up on your instance.
Then the option should appear.
There is one corner case in which the png option might not appear even after installing
Chromium: trying to download a Lookless tile while not having the explore permission. This is by design.