Use the images with transparent background. If the image layer is a Background layer right click on it and select Layer from Background.
Create a new layer and fill it with black using the Paint Bucket tool.
Drag the new layer below the image layer. Merge the layers before saving.
You can record an action while you do these steps, to be able to apply it to any other image with a single click.
https://helpx.adobe.com/photoshop/using/creating-actions.html