I have a similar problem when trying to add a new calendar file to my rainlendar:
(rainlendar2:13375): Gtk-WARNING **: Error loading theme icon 'gtk-cancel' for stock: Fatal error reading PNG image file: out of place IDAT
I guess this problem occurs because rainlendar is statically linked with a different libpng version that the system GTK library is using. PNG related strings in the 'rainlendar2' binary suggest that.
As Rainlendar is dynamically linked with GTK anyway, it should not include its own version of libpng. This way only one libpng version would be used and there would be no conflict.
On the other hand, dynamically linking rainlendar with an incompatible libpng version would also cause problems. Maybe using gtk interface for accessing PNG image (and no need to directly link libpng) would be enough?
BTW I am thinking about purchasing the Pro version… though, not with such a bug.