A manipulated cover must be seamless and believable. The point is to create something people see as a cohesive end result. 

Let's hop into it and see a few things you shouldn't do when creating a manip cover.

Let's begin with a bad manip so I can explain some of the things that you shouldn't do.

This cover was created based on a cover I saw a designer deliver. I changed the title and my images are slightly different, but the setup for the final design is the same.

There is nothing that is pleasing that would make a reader pick up the book and read it.

Let's discuss why this is a bad manipulation -

1) Proportion- If you want images to appear as if they go together then you must consider the proportions of the things you place together. The house is sitting on top of the grass, which makes it quite obvious the images were not meant to go together. The blades of grass are also enormous and the tops are abruptly cut off. The blades of grass are halfway up the guy's arm, if you walked into the lawn you'd vanish never to be seen again it's so huge. The lawn doesn't meet up with the house naturally and doing it this way makes it appear as if the models are standing in front of a huge forest of grass that happens to be holding up a house.

2) Cardboard Models- Please do not ever put a drop shadow on people. It makes them look like cardboard cutouts. There is a difference between putting a drop shadow and outlining as some people do on covers. A drop shadow matches your image exactly because the program simply creates a shadow of the object using it as a base. It also allows you to see the badly cut edges of the PNGs.

3) Quality- There's a difference in the sizes and quality of the PNGs used for the cover models and they don't go together. The male is very fuzzy because the image wasn't good enough quality to be made large enough to go with the female, he also looks like a stalker. The two don't appear like a loving couple. Try and make certain to pick PNGs that go together, don't make awkward pairings.

4) But it's so artistic!- I have no clue why the designer decided to slap that sepia-colored box over the models but it looks ridiculous. It's not artistic, it looks awkward and leaves you wondering why it's there. 

5) Where to put it?- There's very little white space (white space is defined as the empty, open, space on the image) on the background for text. Balancing the author's name on the house looks weird and it's a little difficult to read.

So let's make a better version of this cover, shall we?

These are the images I used to make the new cover.

First we create a blank canvas (wattpad cover size is 512X800) and place the house image on it. The original image was not a PNG so I had to cut it out.

Next we place the image with the grass and size it so it looks appropriate with the house. The next thing I did was add a layer mask to the image of the house, this allows you to erase things and put them back in without a whole lot of trouble. 

What I used the layer mask for was to very lightly erase some of the base of the house so it appears more as if it's sitting in the lawn instead of on top of it. I used a brush with a soft edge set to around 50% opacity. It's a very subtle difference, but one that makes the image look more realistic. You could also use a layer mask to erase the top edges of the grass image since it's sticking out above the house, but you don't need to in this case since the image of the sky can be placed on top of it and it won't show anyway. 

Next we add in the image of the sky.

Now we add the images of the couple. I made them big enough that we can place the image of the man right at the bottom so his missing legs don't matter. 

Next I added a gradient set to 70% opacity so the text can be read more easily. 

I added the text and a PSD coloring and ta-da! A completed manip cover.

For a cover to be considered a manipulation, the images must appear to go together. It must be seamless. You want people to believe it's a single image.

This is not a manip -

To create a manip you must manipulate the images so they go together. Taking a bunch of images, piling them one atop the other, and changing to blend mode on them is not how you do a manipulation, and yes I've seen designers do this and deliver the cover to a customer. Everything is obscured and confused. It's a giant mess all lumped together. You don't even know what the cover is supposed to be. I'd never pick up a book with a cover like this.