This tutorial shows how to use the clone stamp tool in Paint.net to copy portions of one layer to another.
It shows how to change these:
To prepare, save the files used in this tutorial by right clicking on these links and selecting 'Save link as':
1. Open 'Hear See Speak 1.jpg'
2. Select Layers | Import from file...
3. And open 'Hear See Speak 2.jpg'. This creates a new layer and puts this image on it.
4. Do another layer import on file 'Hear See Speak 3.jpg'. We now have three layers, each with a different image on it.
5. On the Layers window, uncheck the check box on 'Hear See Speak 2'.
6. Make sure the 'Hear See Speak 3.jpg' layer is selected.
7. On the Tools window, click Rectangle select.
8. Select the portion of the image on the right to copy. I used the rectangle selection tool for ease, but you can use any of the selection tools. Depending on the image, you may want to use a selection tool which gives a closer crop on the source. This step is not mandatory but I recommend it. If you don't do this, you may accidentally clone over part of the destination layer which you don't want to change and have to undo and start cloning over. 
9. Next click the Stamp tool.
10. Then Ctrl-click on a section to copy.
11. Set the Brush width to 100.
12. Clear the check box for 'Hear See Speak 3'. You should now see the background with a selection rectangle on the right with a round stamp tool indicator.
13. Align the cursor's brush indicator directly on top of the stamp tool's indicator.
14. Click and drag the cursor. You'll see the image from the other layer get copied to the background. Slowly outline the image on the background layer...
15. ...and then fill in the middle by simply dragging the cursor.
16. Repeat for the 'Hear See Speak 2' layer:
- Click the check box for 'Hear See Speak 2' and then click on the layer to highlight it.
- On the Tools window, click Rectangle select.
- Select the portion of the image in the middle to copy.
- Next click the Stamp tool.
- Then Ctrl-click on a section to copy.
- Clear the check box for 'Hear See Speak 2'. You should now see the background with a selection rectangle in the middle with a round stamp tool indicator.
- Align the cursor's brush indicator directly on top of the stamp tool's indicator.
- Click and drag the cursor to copy the second layer.
- Put selection rectangles on straight lines in the image to help hide the transition.
- Don't forget to copy shadows and reflections.
- Use a tripod.
- Use Manual exposure and focusing so there's no change in exposure from one image to the next.