With Photopea, you can let the application load, then disconnect from the internet (or otherwise firewall the app) before loading your image. I just gave it a

With Photopea, you can let the application load, then disconnect from the internet (or otherwise firewall the app) before loading your image. I just gave it a try and I could load the image, edit it in various ways, and save it just fine. If there are any features that require being online, I didn't find them.