Absolutely! You can do this via 'Edit event and tickets':
Open your event
Click on 'Edit event and tickets'
Under 'Choose or upload an image', choose 'Upload' (or if you have previously uploaded an image, click 'Delete image' and then it will give you the option to upload a new image)
Your main header image dimension should be 2048 x 652 pixels.
If you're uploading a separate thumbnail image, this should be 216 x 216 pixels.
When you upload an image, it will initially be used as both the header and thumbnail but you can replace either by clicking 'Delete image' and replacing with your preferred image:
