The image is carried in the 'itunes:image' tag in the tope section of the feed (before the first 'item' tag), however I assume you don't have direct access to the feed. You will need to enter your image into Soundcloud's system in the sam way that you originally submitted an image: I'm afraid I can't advise on Soundcloud.
It's important that you give the image a different file name from the original, or the Store has no way of knowing that it's changed. be careful to observe the requirements: minimum 1400 x 1400 px, maximum 3000 x 3000 ps (and square), RGB, no alpha channel, JPG or PNG, and less than 500kB.