Installing Contextly on Drupal is fairly simple, but does require some advanced Drupal knowledge.
1. The module can be downloaded from the official Drupal project page.
2. The module has a read-me with detailed instructions and required modules.
3. Once Contextly is installed, clicking any of the Contextly buttons will launch the walkthrough. At the end of this walkthrough, you'll get a secret API key.
4. The API key lives in the Configuration Settings in Drupal, under Content Authoring tools (see screenshot below).
5. You can then find other Drupal settings, including what image field to set for Contextly to use to make thumbnail images. You'll also see a link to the main control panel, which lives on Contextly's site.