It works
I tried to change the icons with navtweak2.7 and it works.
Which version of the GTK runtime environment do you using ?
It does not work with GTK+ 2.12.9 !!!!
I have installed the gtk+ 2.14.6-1 win32 on my PC and with this one I donīt have problems to replace the icons.
Already a newer version GTK+ 2.16 is available since 02/2009
SourceForge.net: GTK+ for Windows Runtime Environment: GTK+ for Windows Runtime Environment
Still not burnt a new software CD only tried to replace the icons.