How do i create a 'shadow' on a background image?

Technically, a drop shadow needs some white space around it. Otherwise you won't see the shadow.

In this example, I'm using a transparent background and a canvas of required height and width.  File > New.  See screenshots.

Fig 1.

Next I clicked the new layer icon and used the Shape tool to draw a rectangle filled with cyan color.

Fig 2.

I double clicked on the rectangle layer in the Layers panel to add a drop shadow.  Click on Drop shadow to pen the settings panel.  Adjust the dtop shadow angle to 90 deg and other settings to suit.  Hit OK.

Fig 3.

Fig 4.

Finally, go to File > Export > Export As and save as a transparent PNG.  Hit Export All button.

Fig 5.

keitha70260201  wrote I'm new to photoshop and I'm running the version of Photoshop on my 8.1 Windows PC. The latest version does not run on my current operating system.

Hi Keith

What version of Photoshop are you actually using?  It could make quite a difference on how to make your shadow.

Another way to make a shadow, which is flexible and easy to edit, would be to click the lock item to convert the background to a regular layer.

Then choose Inner Gradient Overlay from the fx menu.

You can start with these settings and adjust as desired.

If you want to change the transition, click on the gradient bar.

And drag the opacity stop to the left to shorten the shadow.

You could create a shadow may ways. Paint one in. Add a gradient into a empty layer as above the back\ground layer.  There are always more than one way to do thing in Photoshop.

You can change the number of pixel you have by interpolating the image using Image size with Resample checked. However, you should only do that when you constrain restrain the resize to preserve the image's  aspect ration so the image will not distort.  If you need to change the image's aspect ratio you need to Crop the image to that aspect ratio discard some image content  or use content aware  scale to distort the image to that aspect ratio you need  ration while trying to preserve the aspect ratio of recognized object in the image you can add a mask to identify areas you do not want distorted.  Content aware scale will not always be able to create a scaled version that will be acceptable to you.