To create one or more stacked images, all you have to do is create a canvas large enough to hold all the images and then drag and drop all the images into one. Assuming all the images are the same size and you want to create a vertical stack
- open all the images in Photoshop.
- increase the canvas size of one image to 200% to hold two images.
- select the other image, drag and drop in to the target image.
- flatten the layers and save.
The key is to match the dpi/ppi of all the source images with the target image. Mixing images with different ppi will result in varying image sizes when they are dropped into the target canvas. You can also use the free transform tool to resize the images once it’s placed in the target canvas. Just be sure to hold down the shift key when resizing to maintain the original aspect ratio.