Well, if your camera takes up to 17W, even if you had 100% efficient converter, that would be 17W/5V = 3.4A at 5V.
Even with your 14W number, that would be 14/5 = 2.8A at 5V.
No converter is 100% efficient, so it's more than that in reality.
Your camera needs whatever power it needs; if you won't supply it, it likely stops working. You can't supply your required 14W or 17W from 5V 2.5A, because 5V2.5A is only 12.5W.
If you limit the input current 5V 2.5A by whatever means, the camera stops working because it can't get the power it needs to operate. If you add resistors, you are wasting even more power from your already negative power budget!
Your options are modifying the camera to be more energy-efficient (quite a big project!), or to accept the fact you need a higher power source - if it has to be 5V, it has to be way more than 2.5A. Assuming your 17W number is correct, and assuming 85% converter efficiency, you need 4A at 5V.
Your camera just is too power-hungry to be powered from most USB chargers. You need to come up with something else.