Follow these steps:

1. Create a new notebook.
2. Open its visual options dialog (Notebook|Notebook properties|Visual options...).
3. Customize the images there (add from .bmp or .ico files).
4. Press 'Save all images to one file' button in left toolbar.
5. Choose My Notes Center folder (possibly C:\Program Files\My Notes Center\...)
6. Save the images to file named main.bmp.
7. Restart My Notes Center (you can close just created notebook w/o saving it)

Now all newly created notebooks will have the images list that you've stored in main.bmp file.
Also you can create this file in any image editor, but note, this file must have (16*n)x16 size, where "n" is number of images, and if you want images to be with transparent background, you should set their background color to "Fuchsia" (R:255,G:0,B:255).

You can safely delete or rename main.bmp if you need to revert to original images list.