Can you post a video of your backflip?
Before you go working on your leg muscles to jump higher, I'd make sure your backflip technique is geared towards height.
Let me explain, a breakdancer I know has learned backflips to incorporate into his dancing, but he doesn't tuck, he throws himself up and back and torques himself around. It looks stylized and cool, but it doesn't get him height.
A decent amount of people who do backflips tend to do them similarly (in my experience, including myself). They may not exaggerate it as much, but they aren't jumping COMPLETELY vertically and then tucking, they're throwing their head back and jumping which loses height.
I only mention this second fact because it looks like, from your picture, you may be doing a fairly exaggerated example of this, and normally the fastest way to gain height / distance is by improving technique.