If you want to gain 15 pounds of muscle its not hat hard to do. YOu look to be in decent shape already so you must at least know the basics. I'd suggest doubling your protien intake and do not stop your cardio but cut it down some. I assume you want this to be mostly muscle weight. Also eat lots of veggies of all colors. Start eating at least 6 high protien meals a day. it also looks like you do not have to worry about body fat at the moment but still avoid foods and products labeled low fat or fat free.. to make these products like that they stip alot of the nutrients your body needs. keep an eye on your body fat % if it starts to raise more then 3-4% kick up your cardo higher. Also hit the gym like every other day and focus on different areas each day, though give about 3 days rest between working out each body part, sufficent rest to the muscle group will allow for faster gains.
As for the guy who said he wants to drop weight you really need to be careful. remember muscle weighs more then fat. So keeping track of your body fat % is important.. if it is high you want to do lots of cardio and a good weight training regimine also eat at least 5 high protien meals . Muscle is the only part of the body that burns fat. and it only does it during cardio workouts.
And for both of you you still need carbs just avoid the high fat cards like white breads and such. they make good taste "White" wheat breads and such now. also try to drink close to a gallon of water a day.
Hope this info helps.