Regarding your questions on the sig I did for SYN Fire Rescue - Use Photoshop elements, or if you can get your hands on it a more advanced version (CS3+). As for images it depends on what you are looking for, I usually google for the image if I believe it probably hasn't been rendered; otherwise planetrenders.net.
If you have any other questions feel free to make contact via my personal forum in the XGG forum area.