Once installed, here's how you use this thing.
Whenever you visit reddit, you'll see image previews under those entries that are images, like so
Without, it would look like this
- Install firefox.
- Install greasemonkey.
- Go to the link redditpreview.user.js a click the Install button in the upper right. If you don't see this button you haven't installed greasemonkey correctly -- in which case return to step #2.