ReincarnatedSaint Good question. Some art, whether or not it’s used for commercial purpose, still cannot be used. It all depends on what the artist set the copyright as. You can search for images on Google, for instance, and set the usage settings to reuse with modification, non-commercial, commercial, etc. Some artists will say directly from the source whether or not it’s copyrighted. Usually, if there’s a watermark, you shouldn’t use it as that often indicates ownership. With that said, most of the time, if you modify the image so that it’s unrecognizable from the original, then you’re okay. Regardless, it’s always a risk to use someone else’s work, and it doesn’t depend simply on whether you use an image for commercial or non-commercial use.
It’s good habit to use royalty free or commercial use images from specific free image websites like pixbay. That site has high quality images. For more ‘artsy’ images such as characters, there are character design websites (kinda like a dress up game) where the artist allows you to use the final character you created. Sometimes, they specify it for avatar use only, or non-commercial only.