There are different ways to resize your image to conform with this guidance, a couple of the more common ways as follows:
In Elements or Photoshop, go to File, click Open, and select the image and make any required changes.
Go to Image Resize>Resize>Image Size and enter 1600 in the width box.
If this results in the height being greater than 1200, change the height to 1200, which would then reduce the width.
Click OK to resize the image.
Note: An image in portrait mode should be 1200 high but might, for example, be only 800 wide and a square image would be 1200 x 1200.
Save the resized image as a different file using the naming convention detailed in the section above (Naming Convention for the Image Filenames); this will ensure your original is unchanged.
You can also resize images in Adobe Lightroom (and some other applications) by ‘exporting’ a new copy of the image, without affecting the original.
This is useful for dealing with a ‘batch’ of images which can be dealt with together in one operation. Other software can resize images.