Hello, that's a card cover. In order to add it, you have to open the card, and click the Cover button on the right.
After that, you'll get many options available. You can upload an image from your computer, choose a solid color or an image from the Unsplash catalog.
In order to have an image as a full card, you just have to simply select the option on the right.
Here you can find the full explanation about card covers on the official Trello blog.
I hope it helps!
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