Hmm, gotta check this out later.

Using the example command given in the article, here's a simple way to wrap it in AppleScript:

set newDiskName to "SecureSparse"
set newDiskSize to "1g"

set shellCommand to "cd ~/Desktop; hdiutil create "& newDiskName &" -size "& newDiskSize &" -encryption -type SPARSE -fs HFS+ -volname "& newDiskName

do shell script shellCommand

Note: The example creates a 1GB disk image on the desktop.

Thanks to [AppleScript Info]