Solutions to improve the aerodynamic behavior of a UAV
You want to move a heavy payload at low speeds. This means that induced drag will dominate the aircraft. If we assume a quadratic polar, the least amount of energy for staying airborne is needed when induced drag is three times higher than friction drag. Assuming a quadratic polar, the optimum lift coefficient at that point is $$c_L=\sqrt{3\cdot c_{D0}\cdot\pi\cdot AR\cdot\epsilon}$$
Obviously, you want a small zero-lift drag, but when you combine this with the high aspect ratio for a very efficient configuration, your lift coefficient will also become very high. You need to be careful to pick the right airfoil, and now Reynolds number effects demand to be observed. Once I know the overall size of your UAV, I can become more specific.

Just an example: If the zero lift drag coefficient is 0.02 and the aspect ratio is 12, the ideal lift coefficient is already 1.5. This is beyond most airfoils at Reynolds numbers below 1,000,000 and indicates a too high aspect ratio. Better add some chord in that case.

Also, some design decisions will look counterintuitive at first. I would strongly encourage you to add wing struts, like in a Cessna 172 or the Dedalus human powered aircraft. They help to save wing weight and thus reduce induced drag.

Don't overdo it with an elliptic planform! This will produce too small a chord on the outer wing and give the design undesirable stall characteristics. Better keep some chord and twist the wing so you will achieve the desired elliptical distribution at the design angle of attack. Better yet, save some wing weight by moving to a more triangular lift distribution – however, for small aircraft the optimum looks still quite elliptic.

The model pictured in your question has a lot of dimples on the fuselage - make sure yours will have a smooth surface.

Also look into improving propulsive efficiency. Use low RPM on a large propeller, if possible. You didn't mention your means of propulsion, so again it is hard to be more specific. Judging from the model, you will use electric propulsion. Use a brushless external rotor with a high pole count (low RPM per Volt).

So please provide more details and then you will receive a better answer.