I recommend using higher quality images rather than a border. due to cropping you will have to precisely move the field to avoid the border, otherwise it will show up when scrolling/reaching the edge, making the border visible. Really ugly.
The wallpaper should be any size higher than 1080x960. Oversampling due to the higher resolution of the image makes the image look more detailed and crips than if you have resized the image to 1080x960 in Photoshop for example, no matter how good it looks then. Because of the cropping even at 1080x960 the 'true' resolution is a few thousand pixels less from the true ~1MP image, resulting in a very slight upsizing by the OS - and even the smallest upsizing without control over the sampling method, might give vague results.