Under Drive operation/Imaging options/Image file you find the settings Compress image file to save storage space. You can choose from the LZNT1 Standard or theBZIP2 process.
LZNT1:This is a compression type based on the Lempel-Ziv algorithm, which is laid out on a real time application. Speed is preferred over the rate of data compression.
BZIP2: is a block type compression process. Compression rate is preferred over velocity.
You can also compress the image after creation under Convert image file in the Tools menu.
Compression increases the time required to create an image file. However, the file then requires less space on the disk.