A long time ago, during 1790 there was a fable written by E. Cobham that was famous by its catchphrase ''Qu'en disent les grenouilles'' (What will the frogs say). The fable itself was inspired by the area in which Paris is now located, back then it was known as Lutetia meaning Mud-Land and was surrounded by swamps. Therefore non-parisien/tourists used to joke around stating that the occupants lived like frogs and toads in the swamp.
Nowadays this comparison is deemed offensive for most french people as it is often used to make them feel inferior, which is believed to be the feeling the artist wanted to portrait by depicting them as frogs.