I've actually got my own background on a custom skin so I will just tell you how I did that
1. Download a custom skin with backgrounds, I recommend FMLovers
2. Find the background you want on the Internet
3. Save it into Paint as the exact same filename as one of the skin's backgrounds.
When you restart FM you get the new background. There's probably a simpler way but this is how I did it. Beware that the image may get stretched