Twistie Image Anomaly on the Web

In views, I use my own twistie image (a plus/minus) instead of the default green triangle. On the web, when I show views that display child documents in a hierarchy, twistie images appears as intended EXCEPT where the twistie expands a parent/child document. Those revert back to the Domino default green triangles.

The same views, displayed in a Notes client, correctly display the intended twistie image.

Does anyone have any thoughts on how I can get the web to display the correct (my) image instead of the default one?