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