I am a GP Premium user. I made a 1500 x 300 pixel header image from a jpeg file that looks good on my laptop. It doesn’t scale correctly on a tablet or iPhone (a lot of both sides are cut off). It seems like I need to make the image less wide. Is there an optimal header image size that I should use? I have not loaded any templates from the site library. I am just using the standard GenratePress theme with the GP Premium plugin. Please do not provide an explanation or answer that is too technical, if possible.