Your can set your Campaign image from the Resource Bank of your campaign.
Select your chosen image using the upload tool on the right of the Resource Bank. Square images tend to work best here.
Choose the 'Approved Campaign Assets' folder.
Click on the Upload button.
Open the 'Approved Campaign Assets' folder and click the Action button next to your chosen image - and then 'Edit'.
Tick the "Set as Campaign Image" box and save settings.
This will update the campaign image in your Campaigns Hub.