We could give you the complicated version of copyright/trademark laws, but it all boils down to a simple rule: if YOU are not the original designer and/or copyright/trademark owner, you are NOT allowed to (try and) earn money from it.
So, if you want to sell downloadable files, you either use images that are in the public domain (you can Google that) or you design your own.
Even if many sellers on Etsy seem to have licenses to commercially use famous trademarks/copyrights, they usually don't. They are breaking the law. They just haven't been caught yet.
So, be original!
Or play it safe by making sure you have the rights to use someone elses images.