Forgive me if there are any inaccuracies here; I'm a Photoshop user and I'm only basing this off
this screenshot of PSP 9 on Google.
Set your first layer to whatever image you want. Set another layer on top of it, this should be a solid colour. See those sliders on the right side of the screenie, with 100, 86, 84, and 76 respectively? Well, for your first layer, which should be your bottom image, leave it at 100, but for the solid colour layer, set it to something less, like 40 or 50. Adjust as you please.