You know what, it looks like the Mionix Naos 7000 would fit your palm grip perfectly. Take a look at it. Best optical sensor available right now, too.
Yeah I agree, the Rival would fit you great if that's the mouse you were using before. I almost decided the Rival was the best mouse for me until I got the Avior, but my grip isn't the same as yours and I actually prefer ambi mice even though I'm right-handed. The Naos might have better build quality, but the Rival's shape is extremely similar to what you are showing here.
Your middle, ring, and pinky all look a bit stressed on this mouse as well. It is not certain you need to change the mouse, just first figure out what's hurting you.