Like all actions in PSP, merging (parts of) two images is very simple once you know how to do it.

For this little tutorial I'll want two football players in my own front garden. Needed: a pic of football players and a pic of my front garden.
Advise: unlike the two images I'll use, it is easier to choose two images with a background in more or less the same hue and saturation.

Now the first thing to do is, of course, cutting the guys out of pic 1. Select the lasso tool, which looks like this:

Once that is selected, open the tool options box (if it's not on screen, just hit the letter 'o' and it will appear).
The settings will be something like this:

although of course you're advised to play with the settings, especially the Feather setting which you may want to be as low as 0 (zero): there's a lot of undo-redo involved in the beginning!

Now with the cursor follow as closely as possible the outline of the part you want to cut out of the original image, left-clicking on the important curves etc. You will see 'marching ants' follow you as you go. When you're done, double-click, and the selection will complete itself. The marquee (the marching ants) will show exactly what part is selected.

If you 'forgot' something, you can add to the selection by keeping Shift pressed while you select the 'forgotten' part: when you double-click now it will be added to the selection. If there's too much selected, you remove the unwanted part by holding Ctrl during the same action.

Now go to Menu > Copy (or Ctrl-C)

Activate the image your selection should be moved to, and add a new (raster) layer: this way you'll be able to make corrections to the new part, without affecting the original photo.
Now choose Menu > Paste > Paste as new selection. There they are, right in my garden:

but they're not on the right spot yet: as long as you keep pasted part selected, you can move it around untill they are exactly where you want them! Now with the Retouch Tool you can make any necessary corrections to the selection. When you're satisfied with the result go to Menu > Selections > Merge > Merge all and you're done!

That's all, folks!