Technically the phrase for that picture is 'murica, which would be derogatory. 'merica is an acceptable shorthand for America and is ok to use. It's all in the spelling, just like other common words that are good or bad depending on where or whom uses them. Ending one such word with an "a" or "er" completely changes its meaning to the point you may or may not get shot when it's heard.

i get your "a" Vs. "er" analogy.... One would DEFINITELY get you shot.. but not by an "Merican".... "'mericans", will shoot you for ANY reason...