Except by mutilating an image through watermarks, there is no way to prevent digital images posted on the internet from being copied & misused. If the potential theft of your artwork is more important to you than getting your artwork out there to be seen & appreciated, then don't post it. I would rather see artists spend more of their time creating art than wasting it going around in circles trying to figure out a solution to an unsolvable problem...

"Every child is an artist. The problem is how to remain an artist once we grow up." - Picasso