personally for me back then i used the image i found in google and for the editing processes i used picarts but now postermywall.com
In able to get a good quality picture go and use pinterest dear ;) Good luck!

Well I download the photos and images I want to us as backdrops and so on and then go to Postermywall.com and make my posters there. but Lunapic.com, befunky.com , photoshop are some good ones too.

I use the app BeFunky...

You could always put requests into graphic shops if you want >.<

I make edits using Deviant Art, they have a nice program called dA Muro. It's not as good as Photoshop, but it really helps with putting transparent images together! PLus you can you use layers and also mess with opacity and such and it's not hard to figure everything out. :)

I use Photoshop for my fanfic covers, but if it's for retouching and things like filter, I like to use phone apps.
Meitu apps are really good for picture editing. BeautyCam is good with retouched. SelfieCity has the best filters in my opinion. PosterLabs has the best collages, but sometimes the font are set so you might have to use something to go over it.

I use a program called gimp. I get my images from google, edit them on gimp, upload on tumblr and then add it to my stories.
You can also request posters from poster shops. There are many on aff. They make the poster and provide links that you can use to upload to your story

I use photoshop for posters

I use pixlr.photoshop and canva sometimes ^^

If you want to make them yourself, you can use pixlr.com. It's very similar to adobe photoshop, or so I heard, and it's totally free!

I'm sorry, but do you mean the posters in the fics?
If so, the posters are made by the authors themselves, or the usual: you can request them from poster shops here on AFF.
I think the ones who make the posters just google the pics and photoshop them (usually using adobe photoshop).

I find photos through Google Image Search, then go through Picmonkey.com to edit it to look the way I want for each story.