I started writing, or rather being brave enough to share my writing with other people, by dabbling in fan fiction. My chosen fandom was Fantasy Island, which was good for me, because the main characters had no real backstory, and there were two-three guests per week with original stories to tell. That meant very little canon to worry about and lots of creative freedom.
The standard disclaimer was about how Fantasy Island wasn't mine, no infringement intended, and lots of names about who owned it at the time (Sony, among others). Credit where due, and all that. Also the "written for love not money" because using other people's work for no profit is totes okay.
In fan-fiction. Depending on the generous nature of the copyright holders. Who generally don't object. But they could if they wanted to. And some have!
However. The idea that one can use someone else's work so long as you don't make money from it has spread to other areas, images in particular.
I was rather distressed to discover this attitude on a writing site, where "please don't steal my stories!" is a common plea. Because some writers there seem terribly protective of their own work, while stealing others without a qualm.
Images. They make book covers for their stories, which is fine, I do that, too. It's fun. It's a great form of procrastination and it helps you feel like a "real writer", I get that. But the idea that "most images are free to use so long as you don't make money from them" is very strong there.
Sorry, kids, it doesn't work like that. Images aren't born out of thin air, they were created, just like your stories were created by you. The rights to use those images belong to the creator and it doesn't matter that it's "on the Internet" -- you're posting your story "on the Internet" does that mean I can change a few names and call it mine, so long as I don't make any money from it? No?
Google actually has a usage search in the image search function, so you can search for images free to use for non-commercial purposes, either modified or without modification. Photographers and graphic artists get to decide that.
Otherwise, it's just as much stealing as downloading movies and music. But it's a little hypocritical to cry "don't steal my ideas!" over your own work while posting it with a cover made of stolen images.