However, if they could move, as for instance in an ionised fluid or gas, then the positive chargers would move in the same direction as the arrow of the electric current.
In a battery the negative and positive charges are generated in a chemical process.
Because of the convention, the power dissipated in a load is positive if the current flow is as shown in the image (so in the load it flows from the positive end to the negative end). So the voltage drop is in the opposite direction to the current flow.
At the same time, the voltage and current have the same direction in the voltage source. But if the power (and energy) lost in the load is positive then the power (and energy) generated by the power source must be negative, to keep the total balance at zero.