Error on startup - "invalid image"

Getting frequent bug reports with this stacktrace:

Traceback (most recent call last):
 File \"whyteboard.py\", line 49, in <module>
 File \"wx\\_core.pyo\", line 7978, in __init__
 File \"wx\\_core.pyo\", line 7552, in _BootstrapApp
 File \"whyteboard\\gui\\app.pyo\", line 61, in OnInit
 File \"whyteboard\\gui\\frame.pyo\", line 115, in __init__
 File \"whyteboard\\gui\\panels.pyo\", line 76, in __init__
 File \"whyteboard\\gui\\panels.pyo\", line 180, in make_toolbox
 File \"wx\\lib\\buttons.pyo\", line 395, in __init__
 File \"wx\\lib\\buttons.pyo\", line 432, in SetBitmapLabel
 File \"wx\\_core.pyo\", line 3473, in ImageFromBitmap
PyAssertionError: C++ assertion \"bmp.Ok()\" failed at ..\\..\\src\\msw\\dib.cpp(148) in wxDIB::Create(): wxDIB::Create(): invalid bitmap

I'm wondering what's causing it -- images not existing?

Reported on:
 - Windows 7
 - Windows Vista SP2
 - Windows XP SP2

Seems to be a Windows-only problem. Perhaps the images aren't being found? I'll have to investigate.