You want to have a thumbnail in front of your offers so your customers can see what they will get in the offer?
When you configure your offers on a new product, you have the option to put the link of a thumbnail directly in front of an offer.
For security reasons, it is recommended to use a URL of an image from your store. This means you should first upload the image to your store, copy the url of that image and use it in front of the right offer.
Click save and there you go!
PS: Make sure you choose a thumbnail that really illustrate your offer to avoid confusing the client.