The correct answer to this open question is the following.
You forgot to include the cartoon. So we had to do some research to find it and help you.
So based on the textual descriptions in the cartoon, the implication that Nast is making by using a bag with a dollar sign on it to symbolize Tweed’s head is that William Tweed -better known as Boss Tweed- was a corrupt and influential politician that spent too much money in bribes to influence politicians and legislators in order to make them support his political interests.
Boss Tweed was the leader of a group in New York called "Tammany Hall," that had so much influence in the Democratic Party.
That is why the cartoonist Thomas Nast wanted to portray Tweed's influence in the United States politics with a big fat man, with a bag of money instead of his head.