What is an .IMG file?
Most IMG files are just a raw dump of the content of a disk. This means that the file contains the exact same bytes as the source disk it is based on.
For LinuxLive USB keys, the .IMG file usually contains several partitions using common Linux filesystems such as ext3 or ext4. These types of partitions cannot be read natively by Windows!
Be aware that using an IMG file as source for creating a Live USB will wipe all your data on your key.
Plus, once you write an .IMG file to a USB device, your USB device may not show its original size in Windows (due to the partitions types) and may not be readable either.
This is why using a .IMG file is not recommended and support in LinuxLive USB Creator will remain experimental.
You can read a detailed explanation on Wikipedia.
To be able to use your key in Windows you will have to delete the partition and/or reformat it.