In the modern age of the internet, it's really easy for an artist to share their work with the public and put themselves out there. This comes with a huge risk: Art Thieves. A lot of people will argue that it's the artist's fault for posting their work online in the first place, but I can't imagine a world where artists keep their works hidden in fear of being stolen from or copied. There are people out there who prey on how easily accessible art is and make it harder on the artist.
There are, of course, the people who knowingly steal art and slap them on t-shirts/prints on sites like Redbubble. The sad part is, they can profit off of stolen art for a long time before someone reports them for copyright. Lots of people won't look into the source of the art beforehand to make sure it's legit when purchasing these products and only think about how cool it looks.
That leads to the people who unknowingly are part of the struggle of art thievery. These are the people who don't check the source of products from and unknown seller or simply don't think about it. These are also the people who don't realize that sites like Pinterest revolve mostly around unsourced and uncredited artwork. More often than not, the original source for illustrations and art is quickly lost on these sites, and no one really bothers to locate them. This may not be direct profit off of the artist, but it's taking the artist's name off of the work, and making it harder for their art to be recognized as their own.
When you are an artist, one of the things you strive to do is make it so your work is recognizably yours. You want people to see your work, and immediately realize it's yours. This is a hard task to accomplish with how saturated the artist's market is, but an artist should always be able to rely on a signature or a watermark. Art thieves take this away from artists and make it extremely hard for underground artist's to be found.
If you want to support artists, the first step is to pay attention to what you're sharing, and make sure the source is easily located!