Image Tool cannot recognize format

I'm trying to resize the Virtual Machine I use for Ubuntu. Every time I try to do so, the Image Tool gets almost through the process and at the very end says that it cannot recognize the format of the image file. The same thing happens on another image I use for Fedora. The VMs themselves work fine...I just can't increase their size. Any ideas why and/or what I can do to solve this?

Thanks.

 

I'm having the same problem with a Windows XP installation. Does anyone have any suggestions?