Simplest is just overwrite the image you wish to replace. You can change the file type, but webp is not supported.
Otherwise please check the readme.txt file in the Mods folder. Create a simple mod using this guide, minimal code is needed just the detailsx.js file and the rest is create the folders needed and copy the files in. Just replicate the basic folder structure, ie an Images folder etc, check the existing mods for a simple guide.
A simple way is copy the MoniqueAva one, remove the personMod.js file and rename and edit the details js file, then delete and add files as wanted. Leave the personMod and update if you want to alter game logic and not just images