When creating an icon (.ico) file for an app or file type it should include images for all resolutions/sizes, for example depending on what view Windows Explorer is set to (List up to Thumbnails) the required icon range from 16×16 to 64×64.
Gimp makes it very easy to create these multi-res icon files, using the following steps.
- Create a new Gimp project (transparent background) of the max required size, i.e. 64×64
- Create a new layer at 64×64
- Draw/import your icon to the max layer size and merge to one single layer.
- Copy the single merged layer and scale to 48×48
- Keep copying the layer and scaling for all required sizes, i.e. 32×32, 16×16, 8×8, until you have many layers of differing sizes all of the same image.
- Save the project as a .ico file, the save procedure should show all of the layers/sizes that will be included with the icon file.
Now when using this file windows will display the correct layer/size as required.
It can also be useful to have a more detailed version of the image for the large sizes and then a simpler version of the image for the smaller images, these can still be in the same icon file and that way your icon is still recognisable in list view.