Mystery Solved

A few days ago we asked if anyone could help to interpret this photograph of two postcards which we know refer to the 1905 Russo-Japanese War. What we didn’t yet understand was how it refers to it. Who is the bell boy on the right? The uniformed horse? Why is he laughing? What does the pump refer to? Russian Reddit users u/sUpport84 and u/agrostis came to the rescue. The explanation below is thanks to him.
First off, these two postcards are part of a set of three as it is taken from a (presumably French) triptych from the period. The whole thing is a mocking take on Japanese ambitions in the Far East.

The bull is Russia, Japan is the frog and the bell boy / servant is Britain. As an ally of Japan, Britain was keen to use her power to curb Russian influence in the region. France, meanwhile was allied to Russia. The artist is poking fun at the pretensions of Japan by referencing the fable of the Frog and the Ox, in which the former tries to inflate itself to the size of the latter and bursts in the attempt. In the postcard the frog asks “Look, aren’t I big enough yet?”. Britain’s reply is “not yet”.
Case closed!