get in front of your wall, as this will act as your green screen. Kind of like a chroma key, it will not show up in the final picture. Take a pose, and have somebody snap a picture. My pose wasn't so great becuase i had to take the picture myself, pressing the shutter with my foot. It isn't advised.
Look for images on the internet or ones that are already on your computer and open them in Paint.Net. Open one picture and use the magic wand tool to select the wall. Mess around with the tolerance level. Then, delete the wall. After you have just your image, or have deleted most of the wall, wand your main image. You may have to press ctrl and click several differently colored spots of the image, but it's usually pretty quick and easy. Then copy the selected image and make a new page. Past the image on the page. From there make sure the image is still selected and use the erase tool while zoomed in to take out any rough edges if there are some.
Step 3: Invert images
Make a new page and past your background into it. Then resize the images you're going to put in to a size smaller than the image before you copy them and paste them in. You can, however, still copy and paste them into the image and use the arrow tool to alter its size, but if you press off of the image, it flattens and any changes will leave checkered delete marks. These can be solved with ctrl z.
Step 4: Extras
Now to add more than just the characters. Using the same steps, make explosions or whatever. Add superpowers, change peoples' faces to horses, and so on.