Как установить yEd-редактор графа на Ubuntu?

Начиная с версии 1.7.22, pngcrush имеет параметр перезаписи.

Попробуйте

pngcrush -ow file.png

См. «Список изменений» для получения дополнительной информации:

Version 1.7.22  (built with libpng-1.5.6 and zlib-1.2.5)
  Added "-ow" (overwrite) option.  The input file is overwritten and the
    output file is just used temporarily and removed after it is copied
    over the input file..  If you do not specify an output file, "pngout.png"
    is used as the temporary file. Caution: the temporary file must be on
    the same filesystem as the input file.  Contributed by a group of students
    of the University of Paris who were taking the "Understanding of Programs"
    course and wished to gain familiarity with an open-source program.