Who doesn't love some striking imagery to drive your point home? Whether you're selling a product or service, trying to communicate complex ideas, or simply captivate the emotions of your users, pictures can do that. Everyone knows they work, and everyone loves them.
Well, everyone except the actual web servers. Thankfully, they have not yet gained sentience, so we don't have to worry about their feelings. Even so, there is a cost to having images take up around 70% of all bandwidth.