How to change vertical order of clips in iMovie

Hello,

I'm trying to make a little movie.

I have moved one strip of video shot with a green screen down from My Media into the editing window below. I now want to add a background image that can be used as the backdrop in the video. I know how to do this because I've done it before.

Problem is it only seems to work if the background image is placed below the green screen footage. And I cannot find a way to make this happen.

The green screen footage is already in the Edit window but when I drag any background image down there it only allows me to place it ABOVE the footage – which means I cannot do the overlay.

It has to be BELOW the footage. But I cannot find any way to move the background image down or the video footage up. It's like once you drag in the first piece of media that is permanently stuck at the bottom.

This can't be right because it's insane. There must be a workaround to swap the two bits of media up and down.

Can anyone help?

Thanks!

Carl