Why do cops, firefighters, mechanics, soldiers etc. put pictures of beautiful women in their lockers, vehicles, etc.?
Same question, wouln't you agree?
So here's a different take on why.
Numerous studies have shown that the average healthy adult male will think about having sex between 5 and ten times a day, depending on the study.
On these boards, the pics serve the same purpose as the lockeroom pics.
The natural tendency to think about sex is combined with a 'thank you for your service' baking the bread to put a positive spin on what men think about in the first place.
A beautiful woman is thanking me. Let's get on with the job at hand.
Get it?
Furthermore, a healthy adult female knows that's how a mans mind works. She knows men are horny. It's in their genes. That's why they think about it 5 to ten times a day. So such a woman wouldn't complain about the pictures either.
Just my two cents.