Once you've optimised your images, you might want to replace your current images in the CMS with the new files.
You don't have to delete the originals. Concrete 5 has a nice functionality called "Replace". This lets you replace any image with a new image.
Login to your website > Search for "file manager" > select the image you want to replace.
Click on the image, a contextual menu will appear. Select "Upload file" > choose the file from your computer you want to replace the current image with, upload.
Image not updated after you save? - Clear your Cache