I just joined tumblr and I have a couple questions....?

Ok so I just made my tumblr today so i basically have no idea what i am doing. I uploaded a portrait photo but it's kinda blurry so I wanted to upload another one but i'm not really sure how. Do I just upload another one and that will replace my current one or do I have to delete my current one first? Also can someone please tell me what a header image is and how I can customize my own background image. And if I don't like how the images or background turn out how can I delete them and add different ones? Sorry for all the questions and thank you for your help :)