The Image Size dialog box opens. I've used arrows and loops to show the items you need to be concerned with.

The first item concerns the dimensions expressed as pixels. They are shown at the top of the dialog. Choose the longer dimension (the Height in the case of this example) and make it 1920 pixels. If for some reason your dimensions are not shown in pixels (in some cases it might be shown in percent), make the appropriate adjustment in the drop down menu.

If the resolution, shown toward the bottom of the Document Size part of the dialog, is not 72 pixels/inch do the following:

1. click the Resample Image box to get rid of the check mark.
2. Go back to the Resolution and enter 72 pixels/inch.
3. Click Resample Image again to restore the check mark.
4. Make sure the resampling algorithm is set to Bicubic, or if you have it, set it to Bicubic Sharper.
5. Click OK and the image will be shrunk to the size indicated.

Now it is time to deal with the narrow side. It needs to be fleshed out to 1920 pixels in black.