Hi TM,
Thanks for letting us know about this. There is actually a bug in the version of the Auctiva Uploader that causes the behavior you have described with certain images, mainly ones that are very large. The company we purchased the uploader from is in the process of correcting that problem and should have a new version out soon - we expect to have this new version integrated into our site toward the end of December.
In the interim, you may be able to avoid this problem by resizing your images to about 1 MB prior to uploading them, uploading them using the HTML Uploader, or perhaps by uploading your images when using an alternate web browser like Safari. While we don't fully support the Safari browser, you may be able to upload through that browser and do the rest of your work through Firefox.
If you have any additional questions or concerns, please feel free to contact our customer support team using the web form on the following page of our site: http://www.auctiva.com/help/requesthelp.aspx
-Mike