I find riding with boots is actually easier, your foot (well mine anyway lol) sits where it's ment to, it does slip a bit.
When you don't have boots on you grip with your toes which is what it looks like your doing, so the stirrup sits to far forward.
I have only ridden with at shoes at the beach though.
Also from the pictures your hands are turned out the wrong way, like your driving a motorbike, turn them up so your thumb is in top.
Your leaning forward, you have to sit back on your tailbone and stay on it. Bring your bum more under yourself, it looks like your about to slid off the other side lol
Your arms are to far forward but if you correct your seat they may just come on it's self.