First things first. I recommend tapping on Juxtaposer's help button. Unlike most help sections on apps, this is an animated slideshow that clearly and effectively demonstrates the app in action. A minute of your time and you'll be set to use the app. Seriously. And seriously, so many other apps need to improve their help sections. Get on that, please.
Anyway. Here you see two photos I took; sweet Johnny on the patio and some Milk Bones.
Juxtaposer has a wonderful stamp tool. That nice image of Johnny can be turned into a stamp that I can stamp it on any area of the image. I can also rotate and resize the stamp and --
Okay, okay, I over did it a bit on these two. I now know the meaning of "Less is more."
Juxtaposer looks best when compositing an ordinary image in an unusual spot, like a construction worker drilling away at a chocolate bar -- reversing big and small. And of course da internetz jez love ta put a funny face on the wrong body. Yep, you'll be doing it too in no time, but when you think about combining images, also think about lighting and shadows to create a convincing look. Make sure that the light or the shadows in one image falls on the same side as the other image. In my image above with Johnny and the Milk Bones, I flipped Johnny so that his shadow was on the same side as the Milk Bones because if Johnny were truly that size, he would fall in the Milk Bone's shadow.
And look below, the construction worker really looks like he's hammering away at that chocolate. Notice how the back foot appears to behind the chocolate and the drill's tip is deep in the chocolate. How is that accomplished? By erasing part of the construction worker's image; in this case, just part of his heel and the tip of the drill are erased, just enough to add that touch of realism. But suppose you find that the sunlight's color is different in your top and bottom layers because the photos were taken at different times of the day or on different days? You should try the adjustment tools like Color Temp or Brightness until the photos match as closely as possible to create an illusion of reality.
I know my image with Johnny and the Milk Bones isn't perfect. Ideally the right side of Johnny should share in some of the Milk Bone's shadow. I may add a partially transparent layer of Johnny, and darken it to better match the Milk Bones' shadow and then I would erase all of the areas that wouldn't be in the Milk Bone's shadow. The result would be a tiny shadow on Johnny's right side on the top most layer, the Johnny you see now, and the Milk Bones in the background layer, which together should produce a more realistic composite.
Well I'm excited to play around with Juxtaposer now. Hope you are too. I would love to see what other people come up with.