How can I edit the custom background I've created?
Click Edit background on the Thankbox that your custom background is applied on. You'll see an edit button (pencil) in the top right corner of it in the background picker.
Clicking on that button will open the editor where you can freely change everything about your custom background, including the image.
Keep in mind that editing your background applies the changes to all Thankboxes that you've used it on.