You will be prompted for selecting two actions:
- Add a blank image
- Import an existing picture
If you choose "Add a blank image", GConvert will ask you to select the icon image's format you want to add: size and color depth.
- The size may vary from 1 to what you want.
- The color depth is set through the pixel format ranging from 1-bit to 32-bit. See the table here for further information.
Once you have set all options, press OK and a blank image will be created. You can then use copy/paste/edit image operations to modify this new icon image.