When I go into an email blast, and upload an image, then upload a new image with the same name, the kilobyte figure updates as if the new image has been uploaded, but it is the old one that remains in the system. For me, this only occurs in email blasts. In web pages, file uploads work properly. NationBuilder support says that this only happens sometimes, and their workaround is to not upload images with the same filename but rather rename the files when you upload them.
In this case we recommend that you upload files with new names instead of reusing old names. This is an issue we will consider taking a look at but don't have a timeline.