For anyone who comes up with this error, if you are trying to remove the images from the parent productparent product at the same time, make sure you have product image placeholders as this error will occur if you do not.
You can check in System -> Configuration -> Catalog -> Catalog -> Product Image PlaceholdersSystem -> Configuration -> Catalog -> Catalog -> Product Image Placeholders
. If you can't see an image next to each of the three options, then you will need to add them. May not be your issue, but was in my case.