I think that B for breastfeeding is better than M for milk with a breastfeeding picture. B for breastfeeding is a direct link between the picture, the sound and the letter whereas M for Milk has the direct link between sound and letter but the link to the picture is very abstract. When things are too abstract they can be a bit harder to grasp for littlies.
For M for mik I would just have a glass of milk.

ETA - D is also for daughter, day, dinner, danger, dirt, dish, Dr and dance

M is also for - man, meal, me, middle, morning, music and museum.