Once you learn the concept you can join as many as you like!
Just a quick note that some digital stamp companies have an angel policy that if you do merge images then you must only use theirs together.
THE SINGLE MOST IMPORTANT POINT TO REMEMBER IS TO MAKE A COPY OF YOUR IMAGE, NEVER EDIT THE ORIGINAL!!!
Simply open your image and press control & c then click EDIT -> PASTE AS NEW IMAGE
There are generally 2 formats of images you may download, one is PNG format and the other is JPG or TIF, they work slightly different so I'll cover both separately. If you don't get things right the first time don't worry just click the back (EDIT->UNDO) button or try again :)
Click the pictures for a larger view.
STEP 1
Next click on SELECTIONS which is on the top tool bar as shown below then SELECTIONS ->INVERT.
Now move over to your 1st image (Sam in Trouble) by clicking on the blue bar above the image. When on this image you need to make a new layer so do this by clicking LAYERS ->NEW RASTER LAYER and click OK.
Next click EDIT ->PASTE ->PASTE AS TRANSPARENT SELECTION and your copied image of Jill will appear on your 1st image.
Your image will be floating so slot it where you would like it and right click your mouse. If you wish to move it again then click on the MOVE tool on the sidebar which looks like a cross above the magic wand. As you can see on the image below the images are over each other but you can still see your 1st image through
Click SELECTIONS ->SELECT NONE
STEP 7
This is the clever bit and takes a bit of practice! Click on the ERASER tool on the side tool bar as shown in the pic below. The eraser menu will appear above the images so look at size, hardness, density etc at the top. Set opacity to 100% and adjust the size of the eraser head, this is trial and error, I chose 18.
You also need to make sure your background colour is set to white which is on the right of the screen (Image below) Click onto it to change the colour. Now working on the layers click onto background (which is your 1st image)
And finished! You can see Jill is now stood in front of the crayon box, I removed lines from the skirting board and put Sams hand behind Jill to hide the crayon! Just remember the eraser will remove whatever layer you select so if you wanted Jill behind Sam you would click on Jill in the layers panel then erase bits of her that you want behind Sam.
Don't worry if you don't get it right first time, just play and have a go. Remember to use a copy of your images and you can always click undo!
In summary
Open File 1
IMAGE ->CANVAS SIZE
Increase canvas to where you want image 2 placing.
Open Image 2
Select MAGIC WAND and click on white outer area of image 2
SELECTIONS ->INVERT
EDIT ->COPY
Click on Image 1
LAYERS ->NEW RASTER LAYER
EDIT ->PASTE ->PASTE AS TRANSPARENT SELECTION and move into place
SELECTIONS ->SELECT NONE
Click on Layer 1 on layers panel (Background)
Use ERASER TOOL