Problems with image sizing

Hi, I am a total novice and I am having a few problems uploading images to my posts. When I upload the image from my computer, it appears HUGE at the top of my post. Should I re-size the image before I upload it? If so, how big should the image be? I want it to be centrally aligned and to stretch the full width of the text. I am using the Sakura 3.20 template. Apologies if you need more information, like I said, I am a total novice. Any help would be gratefully received. Thank you.