It's a complicated meme, the subject appeals to various factors.
1. A (usualy) attractive woman. Perhaps scantily clad. Simple enough to understand in the context of heterosexual male sexuality. Men are visualy oriented, and visual representations of women go a long way with men to direct sexual stimulation. On average, women are more complex, and the image of an attractive male is only used to kick off a more complex emotional response and internal processing that's not as direct as a male response to an image.
2. The gun. This is a bit more complex. It's appeal in combination with the woman has got a couple potential facets to it:
2a. The woman holding a gun creates a visual assumption that she naturaly endorse basic male traits, such as weaponry/defense, machinery/gadgets, independence etc. Things many modern women and the mainstream culture/media try daily to suppress in men.
2b Women, as a general rule, are under-represented in the various shooting disciplines. Seeing a woman draped in the accoutraments of shooting makes her seem unique.
2c Even if it's just a picture, the woman holding a gun provides a subliminal sense for an avenue of approach for a man. Even if in a comitted relationship, men instinctualy are constantly evaluating every woman they see as potential mates, and trying to determine an approach, even if they never act on it. An attractive woman just standing there in a bikini provides no avenue for approach (this is of course all just thereoretical, as she's just a picture, but the mental response in the man is the same.) other than a pretty blatant approach based on sexual attraction, and little else. Despite how rediculous it seems, the model holding the gun, (or tools, or in a hot-rod car, etc. they're all the same) provides the notion that you could approach the woman and discuss the firearms, shooting etc., and provide an initial façade of non-sexual interest.
I provide this analysis in jest, but only HALF in jest.
