Make sure you're logged into Transposit, then click this link to open the Thumbnail.ws settings dialog, enter a name, and click Save.
In the Integrations list, where your key is now included, click Authenticate.
Once you have signed up to Thumbnail.ws, you will receive an API key in your e-mail. Enter the key and click Save.
Now that you've configured your integration, add one or more of the integration's actions to an automation, test the automation, and then add the automation to an incident type.