Hi Himanshu,
I do not believe it's possible to alter images that have already been sent to PrintShop Mail Web, as PSM documents are packed with all their resources when sent to PSW, so even if you replace the original image in the original folder, this won't change anything to jobs that have already been sent to PSW.
So yes, you will have to republish the job if you change the images.
Regards,
Raphaël Lalonde Lefebvre