We've actually started taking and measuring several pictures for measurements, usually three, and then average the measurements.

Other tips:
- Make sure you take the picture from straight down over the snake and the ruler (or whatever you use to measure against). Pictures taken from a slight angle will increase the measurement error.
- Make sure that the ruler and snake are on the same level and that the snake is "flat" against the ground. If the snakes head is raised up, it will make the measurement inaccurate.