add screenshots

doc/integration/trello_power_up.md

@@ -3,6 +3,8 @@
 GitLab's Trello Power-Up enables you to seamlessly attach
 GitLab **merge requests** to Trello cards.
 
+![GitLab Trello PowerUp - Trello card](img/trello_card_with_gitlab_powerup.png)
+
 ## Configuring the Power-Up
 
 In order to get started, you will need to configure your Power-Up.
@@ -23,6 +25,8 @@ Your API URL should be your GitLab instance URL with `/api/v4` appended in the e
 For example, if your GitLab instance URL is `https://gitlab.com`, your API URL would be `https://gitlab.com/api/v4`.
 If your instance's URL is `https://example.com`, your API URL will be `https://example.com/api/v4`.
 
+![configure GitLab Trello PowerUp in Trello](img/enable_trello_powerup.png)
+
 ## What is my Personal Access Token?
 
 Your GitLab's personal access token will enable your GitLab account to be accessed