good things: in the front squat your elbows stay high and in both squats it looks like you have a vertical bar path.
The problem is you wont be able to maintain this positioning when weight gets heavier because your not underneath the bar as much as you could be.
On both the front and back squats your hips are way behind your heels. I can't quite tell from the angle, but my guess is that you forward pointing knees and feet. Open the angle of your feet and that way when you squat and you push your knees open (feel a stretch/tension on the inside of your legs--that's the adductors) you'll be able to keep your hips more on top of your heels and your body will be under the bar rather than sliding out behind it. Look at Greg's videos for demonstrations of these ideas and watch how his hips are over his heels and his knees are pushed out allowing for a vertical spine.