Here's my most used pixel editor (photoshop, gimp, psp, etc.) trick, applied here to face swapping in a group portrait. I have used this technique to get family portraits of 30+ people and have every person in it looking acceptable if not good, even when I have to form a composite from 5 or 6 different images. I'm linking to the files at original size if you want to use them to try it. This could also be used to align pictures for a manual stitch, like for panoramas.
I used to be a fan of the clone tool (rubber stamp?). The method here (mostly) eliminates the need for the undo command and helps perfect the alignment of the two images or parts of the image you want to combine before you even start to brush it in.
The closer the two images are to being the same, the better, so shoot with a tripod if possible and control your exposure as well as you can.
Here is the photo with most of the family looking good, but my nephew to the left of his dad is looking a bit strange.
Here's the area that has that same nephew from a similar shot a few seconds earlier.
Open both files and put small image over the first one as a new layer. In photoshop, you can drag the small image onto the large one or select it all with CTRL+A and then copy it with CTRL+C, then paste it onto the large one with CTRL+V.
With the small image highlighted, change the layer blend to Difference (see top of the image below). What this does is take the difference between the bottom layer and the top layer, so if they are exactly the same color at a pixel, the result will be black. Move the top layer around on the second one until the difference is zero in most places. You have to use your judgement here about what positioning will be best. Usually, I try to align the background pixels because it's usually easier to hide the seam in the foreground objects. Notice how the door is completely black in the image below. Pay attention to where you'll be able to place a seam between the layers where it won't be noticeable—ideally, you're looking for a path you can follow that is completely black and includes the area you need to replace. Once you've gotten it aligned, add a layer mask and fill it with black to cover the top layer completely. The shortcut in photoshop to do all of this at once is to hold down ALT and click the new mask button, highlighted in red below.
Change the blend mode back to normal and choose the paintbrush tool. Make sure you have your layer mask selected and get white and black as your foreground and background colors. Now just paint white on the layer mask everywhere inside the path you chose to follow and it will cover the bottom image's information with the information from the top image. If you make any mistakes or want to refine the edge with a finer brush or anything, just press the 'x' key to swap your foreground/background colors. Anywhere you paint black, you'll hide the top layer's information again. Continue working on the mask until pressing the layer visibility button for the top layer (the eye icon in the image above) gives you the change you're looking for.
Here's what I ended up with on this one.
Good idea for a thread, Pierre. Enjoy!