People ask me "What is your process in writing a story?" Which is usually followed up by the dreaded question..."Where do you get your ideas?" The answer to the first question is pretty much straightforward...though on the surface, maybe not so much. One answer is, however it dang well wants to be written. In other words, whatever works for that project is what I do. The same process may work well for one project...but not for the next. But pretty much there is a method that is shared by all, but maybe not in the same order, is what I should say.
I'm not much of a formula type person. Or much of a form type person either. Though form can be really cool, and there really is one even if it isn't apparent. For last years Nanowrimo novel, I did prep work for it. I did the character sketches/interviews; research on Julliard (setting is like Juliard but not Julliard), and a basic outline. Though I didn't necessarily follow that outline. But I had a basic knowledge of my characters, and plot.
For a short story, especially where there is a prompt like a picture...then brainstorming comes in handy. The problem with prompt writing, is that a lot of people will write the same kind of story. It is really hard to come up with something at times, that other writers aren't doing. It's still a valuable asset to have. There are differences between short stories and novellas/novels, but there are also similarities.
For short stories, I do a rough draft "free-write" where I just get the words down. The structure comes out of that, and then I rewrite/edit from there.
Now as to where I get my ideas... they come from everywhere. Something I read will sometimes spark ideas, writing prompts also do...the news. Snippets of conversations...pictures. Pretty much everywhere. Even people bouncing off their ideas at me, will give me an idea that I can use. But to make the idea unique, that is the hard part.