There is no easy way to convert a jpg into a PCB123 schematic. You would need to recreate the schematic in the PCB123 schematic editor. This means finding or creating all of the parts then layout out the schematic. Once you have the schematic complete you can switch over to the layout and begin moving the footprints into position and routing the traces.
Since you already have the schematic drawn you could skip redrawing the schematic in PCB123 and just start from the layout placing footprints and routing the connections.
Please contact customer support if you have any questions.