Rotating pictures in Windows Explorer

This applies to jpg files.

In Win10, there is now a bit setting in Windows that sets the orientation for jpg files. So the compression is preserved. You will see no file-size difference.

In the older Windows (not sure which one, as I jumped from Win7 to Win10), the jpg would be uncompressed, rotated, and them recompressed -- a very slow process for many large files.

I'm not sure about bmp files, but I believe they are still being rotated and resaved.

You can always use a testfile to convince yourself of this -- simple comparison of the filesize will demonstrated the original file compression is preserved.