Hm, lets see if I can still get it together, it has been years since I read Lacan. All of this should be viewed as just from the top off my head the way I remember/interpreted it.

The phallus for Lacan is the imagined omnipotence and agency of our parents (or other caregivers) as we are a very young child and completely dependent on them, turned into a fundamental aspect of the unconscious. Further, it is some desirable aspect, that we imagined makes our caregiver desire other things more than our own needs. (Think: A mother may not immediately feed a child because she is occupied with something else, like the father.) It is the idea of having something, that can fulfil your needs - but is also intimidating and unpredictable, powerful yet volatile. In sexuation, Lacan argues, the “male” sexuation (note that Lacan already did not completely tie this to sex nor even gender as such, more to sexual roles that develop more broadly but have tendencies) projects this onto the proper phallus, i.e. penis, and desires to control it and use it, i.e. overcome the unconscious concept of castration (the realisation of your own powerlessness and dependency). Whereas the “female” sexuation starts to project onto the phallus the primal desire of getting back the symbolic phallus - fetishising it as something powerful that takes control of you and ultimately will enable you to reach the object of desire. Note that this object of desire to Lacan is a complex concept in the unconscious, and I can’t get it all together (and assume I already misremembered some stuff along the way), but at its core, it is an unreachable, unimaginable part of the unconscious, around which the rest of the unconscious circles, never quite reaching it.

The original image was just an image of a possessive gf holding up a guy by his dick.