Convert image to .jpg which will require flattening the image in earlier versions of Photoshop.

For images 1400x1050 pixels – FIRST make sure all the tick boxes are ticked (there are three of them) and set the resolution to 72 pixels/inch - then if the image is landscape enter 1400 in the width box – leave the height.  If the image is portrait enter 1050 in the height box and leave the width.

The above applies to the lower resolution of  1024x768 pixels by substituting these numbers.