Misconceptions.
After eating, you need to work out, or they will be stored as
body fats. To gain lean mass, load up on protein, like eat 1 ton of
chicken breast, 200 egg whites and drink 50 litres of milk. After
that, go to the gym and find Mr Jim. :D
LOL and just kidding on top.
To effectively build muscle mass, diet and body building regime
is important. The schedule and exercise you do will result in how
you look. Example: if you want something really big, do heavy
weights, eat more protein and cut carbohydrates. No fatty, oily or
sugary food. Skip them if you can. Guarantee to see results within
3 months or so, depending on your intensity.
What exercise to do to build the muscle group, I leave it to u
to google. :D
PS: Cassie has nice back muscle! =D
my BMI is roughly around cassie's BMI... 
however, asking me to choose my diet is hard... very hard
indeed... 