I personally don't like to use flash just because it takes away natural lighting from turtle and its environment.
I usually just turn to Auto mode and turn off the flash when I take picture of my turtle when he's standing still, however, I have to find a surface to put my digital camera onto or try not to move my hand, and when he's moving, I usually turn to fast action mode and turn off the flash, and it turns out pretty good.
I would suggest to have enough light source around the turtle when taking photographs that would definitly help a lot.(such as basking lamp)
hope this help
(however, the result may depend on the kind of digital camera you have, mine's Canon S20, quite old digital camera)