Select your topic in "My Topics" tab to access its Curate page > Manage > Customization.

Topic Icon is the first option you’ll see.

For more on Customization options, check out: http://feedback.scoop.it/knowledgebase/articles/91474-4-1-how-to-customize-my-topic-

Note: 

• If you’ve uploaded a topic icon, this icon will be displayed systematically.
  
• If you’ve not uploaded any topic icon, we will display the image of your most recent post. Each time you post a new content with an image, the icon will be updated.
• If you haven’t uploaded a topic icon and your topic doesn’t have any posts with an image, we will displayed a default “it” icon until you upload a topic icon or one of your posts has an image.