The slideshow feature is a wonderful wallpaper changer. You can also get a Next Wallpaper plasmoid
to force a new wallpaper. Unfortunately the slideshow doesn't allow you to force a specific wallpaper image and there's not an easy way to identify a specific image in a tooltip, but you can right-click on the desktop and select Open Wallpaper image to open it in your image viewer to see which image it is in your collection.
As an aside, you can use an app like Webilder
to get new wallpaper images. It uses Flickr as a resource and you can create custom searches as well as use one of their many "channels" to get images you may like, plus it lets you use images from Webshots if you like that service. It doesn't support changing wallpaper images on KDE but it's simply a matter of someone writing the code to do it. Webilder's dev has said he'd be more than happy to work with someone to write the needed code.