Icon Maker is a simple, Open Source app that can create 32-bit color icons from images that already contain alpha information.
As mentioned, Icon Maker is designed to be simple. The main Window allows you to drag-and-drop images, or you can use the menu to select File, New, or File, Import. Images imported must be the same width and height and no larger than 256 pixels. We discovered a small bug right away when we tried to import a picture with different width and height. It was still loaded into Icon Maker, but we couldn't see it to remove it forcing us to close and restart the app.
Once you imported the image, you can then save it as an icon.
Icon Maker might be a little misleading in the name. Or not. It does create icons, but it's not a "from scratch" creator nor is it full of settings and features. But, that's not the goal here. The goal is to drag-and-drop an image of the proper size and color and quickly convert it to an icon. It does exactly that, effortlessly. It took us just a minute to take our logo, drop it into Icon Maker and save it as an icon. You've seen our website design; we suck at graphics, so if we can do it, anyone can.