1. log into the "Impact Hub"
2. go to the tab "Public Profile," and hover over the logo
3. you'll see the option "Update Logo" as shown in the image. Click there and you can upload a replacement graphic.

Over on the right of that page it shows some constraints and recommendations for size and format:

logo:

• dimensions: 200 x 200 pixels
• size: less than 1 MB
• recommended formats: jpg, png, gif

cover photo ('hero image') -- this is the background behind the logo.

• dimensions: 1170 x 300 (auto-cropped)
• size: less than 3 MB
• recommended formats: jpg, png, gif

Make sure you have rights to any image you post!