Reduce the size of the photo in PSPro (Image/Resize) to the size you want for your webpage (say 480x360 pixels). Then click on File/Save As.. (or Save Copy As..). When you get the save dialog box, make sure the type selected is jpeg and then click on the options button. Set the "Compression" to about 30%. Experiment (with a COPY of the original!) with different degrees of compression until you get the smallest file that still retains the clarity/crispness you want.
I've got quite an old version of PaintShop Pro (v5) but I would think that it will be the same for all.