Look at the bottom of the tool bar, that long skinny vertical bar off to the side. At the bottom you will see foreground and background colors, foreground on top and background on the bottom. We want black in the background. If black is not showing either as foreground or background, click on the little black over white squares in the lower left corner of the tool bar(circled here). This will bring up black and white. If the larger black square is not the background, make it so by clicking on the little two ended arrow at the top right.

Now we can add black to make the narrow side 1920 pixels.

Bring up the Resize Canvas Size dialog by going to

Image > Resize > Canvas Size

This brings up the Canvas Size dialog. Go to the narrower side (the width here) and make it 1920 pixels. Click OK and black (the foreground color) is added to flesh out what was the narrower side.

The final image is shown below complete with black.

Now convert the image to .jpg format.