[KevinH]

You need to rename it in BookBrowser like you do other files, or even use RegexRename to bulk rename them, and Sigil will properly update all links to it throughout the entire epub.

The key is that BookBrowser is where file renaming is done when focus is there to prevent broken links from renaming a file you are actually editing.

You can copy out of the caption or img alt string easily enough out of the html and can see the image in Preview that will sync itself. So there is no need to open a copy of that image just to rename it.

You might also want to try the Image Reports to see filenames and thumbnails of the images in tabular format to figure out new names. You can export that report and use it to walk/rename the images in BookBrowser one after the other.