Follow these steps:
Open your list of raffles
On the left-hand menu, click Raffles and then select Edit/List Raffles.
Choose the raffle you want to edit
Find the raffle you want to update, then click the Edit icon on the right-hand side of that raffle row.
Go to the Images tab
In the raffle edit screen, use the horizontal menu at the top and click on the Images tab.
Upload your banner image
Click to choose or upload the image you want to use as your banner.
Make sure your image is 1280 x 720 pixels for best results.
(Optional) You can also upload custom images for Instagram Story and Feed in the same section.
Save your changes
Scroll down (if needed) and click Save to apply the new banner to your raffle page.