The screenshot below demonstrates a typical workflow, where a label image is dragged from the Image Portfolio palette, onto the Captured Bristle.nib file in this example, which has already been opened in Painter. After dropping on the image, the label imagery is automatically placed on a new layer, where it can be repositioned using the Layer Adjuster tool from the Toolbox palette, or by using the keyboard arrow keys. The file is then saved as an Excellent quality jpg (with a nib file extension), and can be viewed in the Brush Selector, after the original brush library/ category is reloaded.
Note that we are not looking to label every nib file in Painter (unless you really want to), but just the ones which would help identify a certain variant behavior or characteristic, especially in categories which may contain a mixed selection of Liquid Ink, Watercolor or variants which do not add colour of their own for example. It would also be very convenient to drag an image from say the Image Portfolio (or some other template dialog), directly onto the brush variant nib preview window in the Brush Selector in order to assign a label, but this is not currently possible.
Further details and the label libraries are available from this link.