fade effect can also be considered as a erase effect. 

an example of how it is used:

the original picture of korosensei and nagisa is this:

the fade effect is used whenever a picture you want, is in complete/cut off. in this case, korosensei's head is cut off so i decided to put it at the top 

however, his body is also cut off thus, i used the fade effect to try and make it blend into the background.

how to achieve this:

open picsart / photoshop / line camera. in my case, i use photoshop.

you open up the canvas and then place the picture.

after this, take the brush tool/eraser

and then lower the opacity to around 12-20%, depending on your tastes and start erasing 

it'll look weird at first but then once you fully complete it, it'll look nice :) erase the bottom and top that are cut off !

it'll look like this / something similar! 

- Melodie