Hi Gracie,
Based on your description of the behavior you are seeing, it sounds like something on your end (perhaps your browser security settings) is blocking the Active X component from being installed.
The first thing we suggest is to restore the advanced settings in your Internet Explorer browser. This can be done via the following steps:
1) Select "Internet Options" from the "Tools" menu within Internet Explorer
2) Click the "Advanced" tab
3) Click the "Restore Advanced Settings" button
4) Click "OK"
5) Close and re-open your Internet Explorer browser
When you reach the Auctiva Uploader page, the required Active X component should attempt to install automatically and you should see an information bar along the top of your browser that you must right-click to accept the installation.
Once this Active X component has been installed successfully, you should then be able to upload your images successfully when you choose to upload images after clicking the "Select an Image" boxes on the listing creation page.
Give procedure this a try. If you are still having problems, you can file a support case (let 'em know you already tried the suggestion above or they will likely ask you to do it again) or, since Firefox is already working for you, you can use it...your choice.
Hope this helps.