The “Add as Take” option works for me in Vegas 5.
Make sure you right-click it in the Media pool and then do your drag & drop over top of the image to be replaced. Once you release the mouse button, you should see a list of options.
If that doesn’t work, right-click the image to be replace in the media pool and select “replace” from the menu. Then navigate to the folder where the new image is and double-click it. Instant replacement 🙂
Mike