Someone asked if they could change the corner radius of their pictures, and the honest answer was no: every surface had its own corners built in.
Now the Brand tab has a Picture corners row, a slider from 0 to 48 pixels with the number beside it. It sets one radius for every photo the page draws, so the moodboard tiles, the pictures in the motion demos and the asset thumbnails all follow it. Set it to 0 and every picture is square.
A brand that never touches it keeps the corners it had, so nothing already published changed. If you work with your own AI over MCP, it can set the radius for you too.