So from the context, here's my understanding:
A common trend that's observed from a lot of A Happy Death is the treatment of women as mere objects, rather than as human beings. This is portrayed quite well in the beginning of that paragraph:
The natural stupidity which glowed in her eyes emphasized her remote, impassive expression.
The remark, "hello, image" is slightly misleading. The meaning of image used here doesn't mean image as in a photograph, but rather an image i.e. a guise, appearance. You need to remember that this was actually written in French before translation. The phrase used is
Apparence in this context, to quote the Collins French-English dictionary, is
"...guise to refer to the outward appearance or form of someone or something."
Mersault is quite literally greeting Marthe's physical appearance rather than her as a person. If you were to put it more into modern term, it might be something like, "Hello, body". This plays into the character of Mersault who only values his girlfriend for making other guys around him jealous and for being his personal eye-candy (along with other kinds of candy).
I hope this helped!