icns Creator is a macOS application that allows you to easily create individual icns and iconset files from any Gif, PNG or JPG image file. With this tool, you can quickly generate high-quality icns files to use as icons for your macOS applications or generate a single appropriate .iconset file.

It's native, written in SwiftUI, open source. Not telemetry, or any other data collection. Refer to Github Repo to fork, download and install the app.