Oh man, it took me forever to figure out how to customize the icons and the join now button. I noticed another site did it so I knew it could be done.
To change the button, I saved the image join-button-big.gif and took it to photoshop. The images is located here:
To change the icons, I did the same thing. Took the images default.jpg and default_thumb.jpg to photoshop. These images are located here:
Remember, when you save the images, they must be of the same name and file format so when you upload it, the image will overwrite the existing image.
I hope that helps. Also, if youre looking for a message board that has integration with JomSocial, try Kunena. Its Fireboard with a different name and better integration. Let me know if you need anything else.