What is the image in in a station of the metro and what is the function of it?
The image is exactly what the poem gives you: faces in a subway (Metro) stations and petals on a wet, black bough (tree limb).
The image also doesn't have a function. This isn't a poem like the Waste Land where images are often allusions to other literary works or complex metaphors: Pound's poem is supposed to act as a kind of verbal impressionist painting. The poem is searching more for an emotional response rather than an intellectual response.