I haven’t read the story you refer to but in general, there are several ways to have an overlay.
1) you need overlay to be cut off existing background - for example, to cut out table so you can place people behind it.
This you can do in any program that can make such modifications to a picture. I use GIMP …you can find some tips on how gimp works here
but it is not the easiest program to use (it has a lot of features and therefore it might be confusing on the beginning). but there are also many apps.
2) you need specific overlay like owl, lightning, snowball…atc
either again you find a picture having it and cut it out like above
or look for png files - but look on safe pages like pixabay (reasons are explained above - author rights)
Or if you have learnd basis how to draw in any digital painting system you can do it completely by yourself.
In case you cant work in the programs like gimp - go here to the art resources and try to ask - here are always people willing to share and it might be the quickest way how to get what you are looking for…or if you are lucky somebody who can drow might be willing to make it for you. This community is very supportive and I am sure with one or another way you will get the overlay you need.