This
subprocess.check_call(['ebook-convert', sys.argv[1], sys.argv[2], --cover eu.jpg])
blows up,
this
subprocess.check_call(['ebook-convert', sys.argv[1], sys.argv[2], "--cover eu.jpg"])
died
this
Ptotal = " --cover eu.jpg "
subprocess.check_call(['ebook-convert', sys.argv[1], sys.argv[2], Ptotal])
produced the default cover,
this
subprocess.check_call(['ebook-convert', sys.argv[1], sys.argv[2] + " --cover eu.jpg"])
died, and I was laughing before I tried it;
subprocess.check_call(['ebook-convert', sys.argv[1], sys.argv[2], """ --cover eu.jpg"""])
generated the default cover.

OK, I give, what does replacing the cover look like?