I hope this reply is not going beyond the scope of this forum: If you want to rename the filenames themselves before they are imported into Sigil then you can use a piece of free software called Advanced Renamer. This is available from
https://www.advancedrenamer.com/. Use this to rename your files, then when the images are imported into Sigil the names of the images in Sigil will match those of your external filenames.