Wedding wire website picture is blurry

I'm really about to lose my mind. I've tried numerous times, uploading different pictures as the header for my website and every time I do this, it comes out blurry on the phone. I am using my pc to edit the website but I know that most people are going to view the website from their phone, that's why I'm trying to make the mobile version of the website look good rather than the desktop version. I think it is so annoying that you can't just edit the mobile version of the website separate from the desktop version. Like I wish I could upload a picture for the mobile version of our website and then one for the desktop one. I'm super frustrated. There's no place that shows you the dimensions of the header image..... like how am i supposed to guess?? Any help would be appreciated, please... I'm dying here.