Creating backgrounds

Actually, any graphics program. You can use a .jpg picture or a pattern you have created. Are you looking to create something specific? What OS version are you running?
Mac OS 8 and greater (including Mac OS X of course :D) support using .JPG and .GIF files as backgrounds. Anything below OS 8 will either have to be a PICT pattern or you can use a third party app like Decor to use .JPGs and .GIFs. I used Decor on my Quadra 650 (33 MHz 68040 processor) running System 7.6.1 and it worked great.