When sizing the image, remember that the entire width of the newsletter is 600 pixels. So, if you want a picture or banner across the whole page, 600 pixels wide it is. Half the page, you should size that to 300 pixels in width. If you want, it covers a quarter of the page, 150 pixels, and so on.
You may also send the images to help@wiseagent.com, and we can help to resize them for you.