SunFrog is quite different from sites like Redbubble; it’s not a print-on-demand service per se, but actually relies on affiliate campaigns to sell designs. You will find the most success on SunFrog if you already have a social following that you can share your own campaigns to. Otherwise, you will have to wait for sellers to use your designs in their campaigns before you can make money. As such, it’s not the best form of passive income, but as the saying goes, something is always better than nothing.
EDIT: If you forgot your password, don’t try to log in too many times. I just got my account blocked for doing so