Generally speaking, no. Copyright law (typically) means you can’t profit from other people’s creative work. There are exceptions, specifically for some types of “Creative Commons” licenses. Some of those allow reuse of images for commercial gain, but you will often have to provide attribution to the original creator.
However, Creative Commons licenses are the exception rather than the rule. By default, any original work is under the copyright of the owner, and any unauthorized use (where you don’t have permission), especially if you make a profit, would be considered at the very least unethical, and likely illegal.
More on Creative Commons and Copyright here:
If you do want royalty-free, copyright-free, permission-free pictures that you can use for commercial gain, I highly recommend Pixabay. It’s a free image resource, where everything is released under a CC license that allows for reuse and commercial gain, without attribution.