That's the problem I had with a panel earlier this week. With the advice from all the wonderful people here, I trimmed the border so that it was a consistent width all the way around the panel picture. I then zigzagged around the panel. I taped one long side to the floor and forced the other sides straight, taping them as I went. I then lightly sprayed it with water. I was shocked by how much the fabric relaxed. I left it there overnight and it did not return to its wonky shape. I am planning on putting another small border around it just to make sure it keeps its shape, and then machine quilting it.