Are you referring to the gallery upload page? If so, then on that first screen if you don't hit "Preview Image" or "Upload and Save" but just exit out or hit the back button it does cancel it. If you hit the "Preview Image' button then decide you want to cancel there is an "abort" button which means cancel.

If this isn't in reference to the gallery upload page, please be more specific about what area you are referring to.

Thanks