I had the same issue and loaded the image via a WCS. You can get a 30 day evaluation license and install it easily on a dedicated server. What's more, if you decide to buy a license for the WCS all the configurations you made while using the 30 day license will be retained.
If you are doing any sort of implementation where you plan on having more than 2 controllers then the WCS is the way to go. It's the best method for keeping all your WLC configuration changes in sync and it can upload software images simultaneously to all of your WLCs.