If you need a quick way to browse a few folders of images without starting iPhoto or some other serious photo management software, PicturePopPro just might be the Finder plugin for you. It offers right-click access to a surprisingly powerful yet slim and zippy photo browser and mini-manager. Images and directory structures can be viewed in a number of different ways, and a pop-up menu offers the ability to trash or label the image, as well as open it in its default viewer/editor (like Preview or Photoshop). This killer plugin can even be run in a full screen mode, and there are plenty of other options I haven't even explored yet. Check out its documentation page for visual demos, keyboard shortcuts and screenshots of what this plugin is capable of.
PicturePopPro is a Universal Binary and offered as donationware from setnan.org.