I've used a range of programs including snakemeasurer. I've tested them thoroughly along with a group of scientists in the lab. It's possible to get "reasonable" results, but claims of measuring snakes to within 1-2cm are silly (unless of course you're being very careful and the snakes are only 10-20cm long). Generally, for the average Joe user, results tend to be out by around 20-40% and on larger snakes, even more. If you know what you're doing, you may be lucky enough to get results to within 5-10%, but you'll only get this with small snakes (under around 100-120cm, unless you're willing to take the picture from about 5-10m above the snake.