You can use the actual image as it's own placeholder and then browse it via 'replace with graphic file'.
You can use this across every page in a website and only need the one original named image.
This means that you can update an image at any time (and without editing your .web file) simply by uploading a new version of the .png or .jpg with the same name.

(*Currently there is a known bug whereby XWD/X5 will produce a placeholder image, but these can be safely deleted.)