You could simply replace the rss.png file (the current image) in your theme’s images folder with your own rss image. With regard to your image, was there a fixed header width and height given in Admin/Appearance/Custom Header when you chose your new header image?
I couldn’t locate the image folder for my theme. I still managed to replace the image but I’d rather reference it from the correct place. Can you point me in the right direction? I’m temporarily referencing the image from the /upload/ folder. Not the best practice.