What size to make image for page background? Is it possible to reposition in readymag?

I have an about me page on my site with an image in portrait orientation that I want to use for the background of the page. However when I select that photo as the background, it cuts off the top and bottom of the photo, and can be only kind of seen in full when I drag and change the size of the browser window. Is there anyway to reposition the image and make it static // or is there a specific size that the background image needs to be in order for the full image to be seen?