The sort of images most commonly posted to reddit are screengrabs from movies or games with text added for humorous effect. This sort of use has traditionally been classed as fair use under copyright law.
Penalizing reddit for doing so would not result in more revenue for the creators of the source material.
If anything it results in less. Memetic sharing of snippets of content leads lots of people to get interested in the source material, seek it out and pay for it.
Penalizing reddit for doing so would not result in more revenue for the creators of the source material.