Wow. Two way syncing is very complicated, mainly because you have to consider all sorts of unusual situations. Two way syncing is not complicated for the simple situation you describe, but consider:
1. I delete one picture on device A, but not on B. When I sync, do I move the missing picture from B to A, or remove it from B and leave A alone?
2. OK, #1 was easy. Now I delete one picture (X) from B and a different picture (Y) from A. When syncing, do I: 1) delete X and Y from A and B; 2) move X from A to B and Y from B to A; 3) move X from A to B and leave A alone; 4) move Y from B to A and leave B alone?
3. Now I have three devices. I delete one picture from B, but not A or C. When I sync B and C, do I delete the picture from C? If so, then what if I delete a different picture from C, independently add a picture to A, then sync A and C. OK, C now has the new picture. Then I sync to B. What pictures do you want on B?
4. I delete one picture from A, a different one from B, a still different one from C. I add a picture to A, add a different on to B, and rearrange C. Now I sync A to C, then delete a picture from C but not A. Then I sync A to B, then sync B to C. Then I sync A to C. What pictures do you want on each device?
You can come up with unusual situations by yourself - give it a try! It's fun!.
True, for each situation, you can come up with a rule that covers it. But the difficulty is that the rules are complex (because, as we have seen above) the situation is complex. Thus the user becomes unsure what will happen to their pictures. This is true even for mega-ultra-power-tech-users like yourself.