Self Service Icon Maker is a quick easy way : https://github.com/mm2270/Self-Service-Icon-Maker

Rich T.'s excellent Icons.app is still my go-to.  https://github.com/SAP/macOS-icon-generator

okay so once they are created how are they applied? 

From any policy's Self Service tab, click the Upload Icon button, and choose your icon file.

Honestly, making them with Preview is so simple that I can't imagine using an app for it (aside from Preview 🙂

But hey, I surely appreciate all the folks that build stuff like that for the admins!

I use the Application / Contents / Resources / (usually) appicon.icns

Double click the .icns file to open in Preview

Select File / Export ...

Export to .png file format

Keep it the same icon as the app so the end users know what to look for in /Applications
Also, nothing to install

I typically just click "get info" on the app I need an icon for, click the image in the window and Command+C to copy it. I then paste the image in to a photo editor to set it to 512x512 and save it as a png. I very rarely get fancy and make my own icons.

May also want to check out - https://sveinbjorn.org/iconscanner

Open icon from search results in preview, export as png