Open the image on which you want to replace the original background to black fill. Create a duplicate layer (Duplicate layer) and make the layer with the original image invisible by clicking on the image layer in the palette, the icon of the eye.
Now on the toolbar, select the Background eraser tool (the eraser allows you to erase the background, select the eraser size to match your image, select the sufficient stiffness and in the "Tolerance" set the value to 40%.
Then carefully start to erase the background around your object that was previously hovering over it a mouse cursor with the eraser and right clicking. When you remove the main background area, reduce the brush size and reduce the level of hardness to more accurately cut the background color for the silhouette of your object.
Completely remove the background around the object, select the fill tool (Fill) on the toolbar and creating a new layer, fill with purified from the background region as black. Zoom in and carefully examine it for signs of flaws in the pre-treatment of the background region.
If you see remaining from a previous background of dots and spots, again take the background eraser and go to all unwanted stains.
As you can see, to create a simple black background is not difficult, and if you need to, you can always replace it with another background image.