An alternative approach would be to change the image path for the style in Style Property Groups -> General -> Settings: Path to images
That way you won't lose the original icons if you ever need them in the future (although you will always have a copy anyway in the zip).
You will still have to upload 2 icons though as Shelley described.
And depending on the sizes (as not all icons are the same size) you may need to venture in the following:
Appearance >> Styles >> Your Style >> forum List >> Node Icon and adjust the width and height accordingly. The width and height boxes are located in the bottom left. This may avoid any clipping of the images if they are larger than the default icons.
following this path (shouldn't be to hard to locate). styles/default/xenforo/widgets/ or you may have a custom folder for a custom style just replace /default with your custom style folder and follow it from there then drop the images in.