Resize Original Image File Already Uploaded?

Hi guys,
I have seen numerous plugins that will resize your thumbnails, or images AFTER uploading, but what about images that are already uploaded on a site?

Is there a plugin to resize the full-size/original image files to something smaller that are already uploaded to your website?

I know there is the ever so painful task of finding all the images on the computer, resizing them in Photoshop, and re-uploading under the same name… but I’m hoping there is something I can do that is easier than that!