Can confirm LarterStyles is 100% correct. Workaround to change image in an existing deck is to restart the game. The first time the deck builder is opened it will work as intended to change the deck image. Subsequent openings of the deck builder cause a subtle but increasingly large/defined orange glow sprite around the deck image, which appears to be preventing the deck image from changing.

Same problem, just started for me today. When I import a deck, the image of the deck is set to a basic land. I drag the card that I want into the deck image area, that has worked for me hundreds of times, and the image doesn't change. Still stuck on a basic land.

You can change the deck image as expected if it's the first time you've opened the deck builder after opening MTGA (i.e. you can change the deck image if you close and reopen MTGA).

Note that there's currently another bug which makes the orange "glow" behind the deck image grow larger each time the deck builder is opened. This is seemingly happening because a new layer is being added to the UI each time the deck builder is opened, which sits in front of the deck image, and therefore it's actually blocking you from dropping a card onto it.

I think I'm having the same issue and can supply some additional information.
I can change the deck image on decks I created a while ago, but when I create a new deck, even when only using cards from older sets, I cannot change the deck image.
After attempting and failing to change the image on a new deck, I can no longer change the image on any old decks until I close the MTGA client and start it again.

Windows 10, Standalone MTGA installation

same here, even after the first patch. It let me change the image only once in a while, cant see a pattern in it tho.

Same