The icons used in Haiku® are a special type of vector image called Haiku Vector Icon Format and called HVIF for short. This format was created by Stephan Aßmus (stippi), specifically designed to store icons.
The HVIF icons in Haiku’s repository are not trademarks. Instead, they are provided under the MIT license and are free to re-use or modify in any way. The only provision is to mention the origin of these icons somewhere in your documentation when distributing the icons. See the following mailing list posts for reference: HaikuPorts-logo, Splash-icons-license.
As the icons provided with Haiku are a specialized format, a specific application is required to edit them. The tool to use is Icon-O-Matic, which is included in the base image. Icon-O-Matic is capable of exporting HVIF icons as SVG; a single 64x64 PNG; a PNG set of 64x64, 32x32, and 16x16, and several other formats.
If you are creating your own icons or modifying existing ones, the following pages will be useful: