your question is, in absolute terms, impossible to answer: you haven't included all of your code (what are the types for the variables pixmap and mask?), nor have you provided the exact error message ouput (which argument is failing gtk's assertion attempt?).
1) gdk_pixmap_create_from_xpm() requires a filename containing xpm data. the file you have provided is .png format, not the same.
2) gtk_pixmap_new() is deprecated, rewrite your code using the proper gdk equivalent.
and generally: at the very least, make all arguments you provide to routines match exactly the types required for each routine (as defined in the documentation), this will get you a long way towards solving your problems.