GIMP - GIMP is a multiplatform photo manipulation tool.

Icon Converter by RedKetchup - Convert a PNG, JPEG, WEBP, HEIC, GIF, TIFF, BMP, or SVG image to the ICO format. Customize the icon background and rounded corners. Create a compatible Windows ICO file from an image.

Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.

IcoFX - IcoFX is an all-in-one solution for icon creation, extraction and editing.

MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.

png2ico - Program to convert .PNG files to a Windows icon resource, such as used for website/bookmark icon favicon.ico. Available for different platforms.