I don't know if it's a firmware update or if it's always been like this (I only got my HitBox about a month ago or so) but for "sweeping motions" like quarter circle and half circle inputs you don't need to input the diagonals. Sometimes it will happen anyway if you are trying to input your moves too fast but it's all good.
The way I do it is:
I assign each button a number like:
Back = 1
Down = 2
Forward = 3
Up = 4
Then I write a "notation" for what it is I'm trying to do. For example, Sonya's Kartwheel would be 2,3, FK.
After this, I go into the lab and
very slowly (so slow that the move doesn't come out) tap out 2,3. I will also use my right hand to tap out a rythm. The goal here is to have a steady rythm and to train your hand to get used to the input so that eventually you won't have to think about it. That is the reason for starting so slow. It's all about the accuracy. The speed will come naturally the more comfortable and "confident" your hand becomes with the motion.
Once you feel comfortable with the motion, start speeding it up little by little. As soon as you drop an input, slow it back down a tad and continue.
Another way of thinking about it is basically the same but instead of using numbers, use your fingers. So as an example, the notation for the left hand inputs for Sonya's D4 MS F1 (on the P1 side) would be:
Middle Finger (MF), Ring Finger (RF), Index Finger (IF)
Then go into the the lab and practice just doing 2, 1, 3 (MF, RF, IF) over and over.
I have used this technique for MK, Skullgirls and SFxT and it works in all of those games.
One thing I noticed in your picture is that you tuck your left thumb under your hand. It is better if you don't. The default hand postitioning that they teach is to have both thumbs over the Up button and it's a good thing. I personally use my left thumb exclusively for jumping, but as some of their vids show, sometimes it's not a bad idea for the right thumb to have access to the Up button as well.
Hope this helps