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 foto but it's kinda blurry so I wanted to muat naik another one but i'm not really sure how. Do I just muat naik another one and that will replace my current one atau do I have to padam 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 imej atau background turn out how can I padam them and add different ones? Sorry for all the soalan and thank anda for your help :)