This email sent by a senior military analyst who is working on a project to better integrate women into all military positions states that the only way to get ladies interested is to make ensure that the women selected in their ads "are pretty but not too pretty." Here's why:
In general, ugly women are perceived as competent while pretty women are perceived as having used their looks to get ahead.

There is a general tendency to select nice looking women when we select a photo to go with an article (where the article does not reference a specific person). It might behoove us to select more average looking women for our comms strategy. For example, the attached article shows a pretty woman, wearing make-up while on deployed duty. Such photos undermine the rest of the message (and may even make people ask if breaking a nail is considered hazardous duty).
The photo was originally published in a feature written by General Robert W. Cone for the Association of the United States Army magazine. The article had  explained the Army's program "Soldier 2020," which is an attempt to make sure women are a part of "all the remaining combat arms career fields":
We found that most men who had worked and fought beside women expected them to do well in combat roles. Some work remains to be done in order to convince men with little or no experience serving alongside women that they will perform well in new roles, but I am convinced that those with concerns will change their views.
While we get that the memo has the best intentions for women's advancement in the armed forces, it still seems a little unfair that even in the military, women are constantly being scrutinized by how they look instead of their ability to get the job done.

[Politico]