Updating photos without messing it up

Hi there, I am fairly new to WordPress in general.
I have recently been handed over a website so that I can update all the photos they have.
All I am doing is uploading exact replicas of those photos but these new ones have a watermark on them. The person who uploaded the original ones has this catablog plugin setup and I want to know exacty how I’m supposed to do that without messing up the “shortcodes” (I think that’s what they’re called) that he has setup.

When I log in to the back end, I see:
> Library
> Add New
> Galleries
> Templates
> Options
> About

My thinking was just delete the pictures from library and add the new ones with the same name the originals had? But I am obviously not sure. Also, this might sound dumb but do I need to upload the pictures I am going to add to the file manager in the host website or are these pictures just handled through WordPress?

Any help is greatly appreciated, Thank You.

https://wordpress.org/plugins/catablog/