Yeah Mitch, I'm with CUBE on this. You don't look underweight at all. Your torso and chest have definition, your legs look proportionately muscular, your shoulders are broad enough that you can be that weight and look really good.
If I looked that good, I wouldn't push it. I'm a big believer in eating real foods to gain weight, not protein shakes and the like because the body can absorb nutrients, minerals, and vitamins from food far easier than from artificial supplements. If you're serious about doing this, I highly suggest having a consultation with a nutritionist. Together you will estimate your daily calorie burn, level of activity including muscle building and cardio, and then put together a plan that will give you foods your body can process so you can gain weight safely and healthily so you will avoid heart disease and other maladies later in life.