Why should you care? Not only does a nice picture make your diaries look nicer, but content with images shares better. And I think it's safe to assume that you write in order to be read by as many people as possible, right?
And it's super easy, too. When writing a diary, look to the right column to the box that says "image queue". You can upload your own (legal) image, or you can click on the "Image Library" link and browse from an image library now over 70,000 strong. If you want something more visual, here's our tutorial.
DK5, targeted for a July release, will make it even easier to publish diaries, and that includes an even more intuitive image workflow. But what we have now is already pretty good, so don't be afraid to start working with pictures to take your diaries to a whole new level of awesome.