You only need to mimic the existing icon sets in share/config. Name the new set e.g. images_my. The part ‘images_’ is mandatory, the part ‘my’ will be the string shown in the dialog inside LibreOffice. With ‘mimic’ I mean, that you have to keep the folder structure, file names, file size and file format. But what an image file shows, is up to you. The package has a file ‘links.txt’ which holds the references for those images, which are the same but used under different names. You need to update this file, if you will use one image for different commands.
I would start with an existing set and copy it to images_my. Then go into the package and replace all those images, for which you want different ones.
Be courageous, you cannot damage your LibreOffice. You only need to choose your set in Tools > Options > LibreOffice > View, and if it does not work go back to one of the predefined.