Set background image to black

I guess masking is needed for this task. You just need to select the image well and then need to change the background.

Lucie

You have not shown your layers panel but it looks as if you already have the layer with transparency (or with a mask) so we can see the transparency chequered grid.

That being the case, all you need to is go to Layer - New Fill Layer - Solid Colour - then choose black.

If you drag it in the layers panel so that it is underneath your image with transparency - you will get what you want.

Note if the layer with transparency has a lock symbol on it, you will not be able to change its position in the panel. Just click on the lock to remove it.

Dave