(Caveat: none of us here are doctors or physical therapists, so take what we say with healthy skepticism.)
Do you have your palms/wrists resting on some surface? If so, I recommend trying to avoid that while actively typing, to see if that might help. From the picture it looks like you are slightly extending your wrists (bending them upward), and could also be pressing on a nerve or other soft tissue. It might help to bring the keyboard slightly closer to your torso (e.g. by scooting your chair slightly forward). Try to angle the top of the keyboard so that it’s roughly parallel to your palms/forearms when your wrists are in a relaxed, neutral position.
If you just lightly rest your fingers on the home row, without typing, relax as best you can, and concentrate for a few minutes on how your arms and hands feel. Can you feel any strain or muscle tension, anywhere from your fingers up through your shoulders and back? If so, is there any way you can change your position which reduces that tension? Ideally you want to have as little static load as possible, if you plan to stay in the same position for long periods of time (not just for typing, but for any repetitive/long running activity).
One thing I do is take frequent tiny breaks (ideally for at least a few minutes every 40–60m) and switch positions from time to time, which relaxes any muscles which have been under static tension. I have an adjustable-height desk and a highly adjustable seat, so I sometimes sit on it high saddle style, sometimes sit on it like a regular chair, occasionally tilt it back and lounge a bit, sometimes stand, sometimes walk around thinking for a bit, sometimes take a laptop or book or pad of paper over a couch and lie down for a while, etc.
One last thing is: it’s possible to put extra load on muscles by tensing two opposed muscles at the same time, even if the position itself doesn’t require it. Sometimes when doing an unfamiliar activity, if I’m focusing hard on getting the motions right, I’ll find myself unconsciously tensing up the muscles in that area.