Can I resize the javascript image they supplied? eg. <img width="specify my own width" height="specify my own height"> The image that are available in cj are in the size of 600x800 pixels.
Ask the advertiser. You'll be surprised at how lenient they can be. Honestly though you'll probably be fine resizing unless they expressly say that you can only use the links they provide you.
You can resize images or even replace them with your own. The most important thing is to keep the URL intact.