If I upload my private template image to "Image Storage", which format should I select, RAW or QCOW2? Please tell me the difference between the two formats as well.

When you create the template image in RAW format, the data is not compressed and the image is uploaded to "Image Storage".
If you create the template image in QCOW2 format, the data is compressed and the image is uploaded to "Image Storage".
QCOW2 format tends to take less time than RAW format when it is uploaded to "Image Storage".
However, when you create the image in QCOW2 format, it sometimes takes long time to compress the data.

Reference document
FAQ - How long does it take to create snapshots of virtual servers and images using the upload function?
Service Descriptions - image-storage - (Supplemental) Detailed specifications of image copying (Common for Private Template/private ISO)