Open your image in Elements. I am using Elements 2.0 here since it is likely to be the earliest version you are using.

As a rule it is a good idea to work with an image that is not in a lossy format such as .jpg, if only because it is a very good idea to regularly save your work and every time a .jpg is saved it degrades. If your image is currently a .jpg convert it to .psd or .tiff. You can do that by going to

File > Save as

and selecting your choice from the drop down menu.

Now resize the image by going to

Image > Resize > Image size.

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 pixel dimensions. 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 pixel 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

• Click the Resample Image box to get rid of the check mark and then go back to the Resolution and make it 72 pixels/inch.
• Click Resample Image again to restore the check mark.
• Make sure the Constrain Proportions box is checked.
• Set the resampling algorithm to Bicubic, or if you have it, Bicubic Sharper.
• Click OK and the image will be shrunk to the size indicated.

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