How about a poster with a bfeeding mother and standing behind her on either side, looking cheerful, maybe with a hand each on her shoulder or behind her back, an older lady (mother or MIL) and a man (I know not everyone has one in their life but for the sake of argument...) and a slogan like
'Support her choice' or
'Support her decision to breastfeed'
Or one with the woman by herself and
'Give the best of yourself' or
'Give your baby the best start in life'
'Best for you and best for your baby'
...and underneath mention some of the health benefits. I would restrict the mention of health benefits to one or two per poster, maybe one benefit to the mother and one to the child, but make several slightly different posters with different benefits on them.
I originally thought up the 'Give the best of yourself' slogan for a Spanish poster (was just imagining a Spanish campaign, there wasn't actually one unfortunately) and it worked better in Spanish because 'breastfeed' is 'dar el pecho' - which translates as 'give the breast'
So it would say in translation:
'Give the best of yourself, give breast'
I wouldn't think it would be necessary to mention the word 'breastfeed' so long as the images are simple and powerful.
I think we need to see large posters of women breastfeeding on bus-stops and hoardings all over the place, all the time to normalise the image of the breastfeeding mother and counteract the other images of breasts as a purely sexual object for the enjoyment of men, which we are bombarded with.