iPhoto - Removing image files that weren't taken by a camera
You can create a smart album to divide your pictures into the ones you wish to keep &amp; the ones to delete. There are several metadata fields you can use to create an album that divides your images - you should likely start with the Camera Model field.

So, to do this,


Select New Smart Album in iPhoto
Under Match the following condition, select Camera Model is (whatever camera you wish to retain).
If you have several cameras create more than one condition, and change the "Match all of the following conditions" to "Match any of the following conditions."


Depending on your setup, you can reverse this search - instead creating a Smart Album that contains just the images that weren't taken with any of your cameras, and then deleting the downloaded ones...