Thanks for the feedback PackRat! And sorry for the delay! A few (read many) things kept me away from computers and this forum!
Your observation is correct! A few image viewers failed to open images downloaded by pmxkcd. It happens because of a bug first reported in 2007 and guess what?, it is still open!eog does not sniff mime type
I have updated pmxkcd to fix this issue! Thanks again RatMan!