"file could not be opened because it is empty"

This is not the first time I've gotten this error message after using iPhoto. Before I post photos online, I edit them in iPhoto. Then, for the ease of uploading, I transfer them all to a folder I create on the desktop, and upload directly from there. A good chunk of photos then become unreadable and "unuploadable" - when I open them from the folder it says that "file DSCN0691.jpg could not be opened because it is empty", and they don't upload.

What is this problem? It's really bothersome and prevents me from uploading photos that I want to share.

Any tips? Thanks! 🙂