Absolutely! You can do this on Edit event and tickets:
Navigate to your event via the Events section of the Ticket Tailor control panel.
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 an 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: