If the Slack integration has been configured, you should see the Share via Slack button on all Dashboard widgets that provide an image export (besides the Pivot Table). If you don't see the button, you probably need to configure the Slack integration - read here for more details.
When you click the Share button you'll be presented with a dialog to control the channel and message. The message is optional but will help provide readers with the context to make sense of the image.
Once you click Confirm you should then see the message appear in the selected Slack channel.