i did electrical engineering, but throughout most of it i wasn't really paying attention, so just take the following as rambling.
Some devices have a fixed resistance, like torchlights, and will accomodate a wide range or voltages b4 being damaged. V=IR applies in this case - a lower voltage just gives you less light.
Other complicated devices (like cameras?) have stricter or fixed power requirements to work. P=IV applies. If your voltage drops (significantly) and if your source is able to, a higher current is drawn. More heat in components, possibly shorter lifespan.
Ok, lah, i'm just scaring you. i don't think 7.4V and 7.2V will make much of a diff, provided the 7.2V can deliver the same current, at least. Most rechargeables are rated lower than alkalines - 1.2V against 1.5V, if i recall. Usually works just fine.
[OT: i use an electric toothbrush. i'm *quite* sure the brush is more powerful with normal alkalines than NiMHs. i think bcoz of voltage diff. Anyone using electric toothbrush w similar experience?]
Most likely the camera will have internal DC to DC convertors to step up or down the battery voltage to 2 or more different internal working voltages for different parts of the electronics.
In that case, 7.2 or 7.4 really does not matter much. The camera may work down to 6V (my CP 995 can work down to 5.6V from external battery source). The lower the voltage, the higher the current draw.
From my limited understanding about LiIon batteries, they are usually made up of series or parallel connection of a number of 1.2V cells. That's why LiIon (and NiMh) battery pack voltage ratins are usually in multiples of 1.2V.
6 x 1.2V cells in series will yield 7.2V. This is the nominal voltage rating.
When freshly charged, the battery voltage may be as high as 7.4V and dropss quickly to 7.2V as it starts discharging.
7.2V or 7.4V batteries may very well be the same thing. The difference is just a choice of rating the nominal voltage or the fully charged voltage.