This is a special function of the icon editor that works with a third-party image editing tool such as Paint, Corel® Paint Shop Pro or Adobe® Photoshop. You can specify your favorite bitmap editor in the Environment Options; otherwise the default program associated with bitmaps will be used.
As GConvert does not feature a bitmap editor, this feature lets you use your own favorite image editor. By clicking on "Open Image In Editor" in the icon editor, GConvert will save the current selected icon image to a temporary bitmap file and then launch your default image editor. Normally your image editor automatically opens the temporary bitmap file and you can modify it.
After modifying the temporary bitmap file, save you changes and close this editor. GConvert will normally detect when the editor is closed and then ask you whether you want to save changes made to the bitmap file back to the icon image. If you answer Yes, GConvert will open the temporary bitmap and use it to replace the current icon image. The temporary bitmap file is then deleted.
Ensure that the bitmap's background color remains the same as the one of GConvert, and secondly ensure that the selected icon image in the icon editor is still the same. When reading the bitmap file, GConvert will create the corresponding mask using the transparent color and finally update the current icon image. Thus the transparency should be preserved.
Note: 32-bit icons require special care as they also contain the alpha channel in addition to the mask. In this case, the transparent color is not enough. That's why GConvert can also include the alpha channel in the temporary bitmap file. Therefore the resulting bitmap will have a width*2*height size and it should then be split into two width*height bitmaps, the left one will be actually the image bitmap and the right one the alpha channel bitmap. See the following example:
will generate this bitmap
This lets you edit the alpha channel while you are editing the image. In this case, when you save the modified image, be sure to both save the image and the alpha channel again.
Note that this behavior can be disabled in the Environment options using the "Include alpha channel" option (though this is not recommended).