If you want to change the brightness to blend your two images together, PSP X has a tool for this. Its called the lighten/darken brush. Use it by pushing L. now use it as a brush to lighten an area, if you want to darken use the right click instead of the left
If you find that the change was much too drastic you can change the density and harness in the tool's option menue. (it has two presets included to help you; "small blend" and "large blend"