> I would like to be able to replace the drive and path for "Image Picka" in Filename pattern.
Unfortunately this is impossible without a native app. The browser only allows extension to download files into the download folder.

> I want to set the index number to a fixed digit. (Example: 001)
Here is the example to zero-pad index number:
https://github.com/eight04/image-picka#use-expression-in-filename