How icons are stored
The icons you see in the Icons pane are really separate .ico (for icon) or .bmp (for bitmap) image files. These small files actually reside in the treepadlib.tpz file in TreePad's installation directory, where they are compressed into a single file.
TreePad Zip (.tpz) files are very similar to other Zip compressed files. You can view their contents and extract their icon and bitmap files using any zip compression program such as Freebyte ZIP. Please note that although you can extract icons from a .tpz file using an external zip program, it is not possible to add or replace icons using this method, nor can you create new .tpz files with an external zip program.
Please use the TreePad Icons menu for creating icon libraries.
The Icons pane by default shows the icons contained in treepadlib.tpz, you can replace it with another icon library file such as my_icons.tpz in the diagram above (see Icon libraries).
Assuming you are using treepadlib.tpz for your current icon set, you can add extra icons to the Icons pane using the Add new icons command. You can also delete icons from the pane and thus from treepadlib.tpz.