You ought to make use of the Terminal.

Put all of the .img files into a folder in the root directory of your hard drive. We'll name the folder "image"

Now, open the terminal and type this command:

In less than a second, all of the .img files will be gzipped and can then be safely transmitted about the Internet. Bonus-- users of every major computer platform will be able to extract .gz files, as opposed to the formats mentioned earlier, which are rather rare on systems other than the Mac.