If I'm sexing visually from just a picture, the margin of error does go up. I have to actually hold the snake and examine it in person to get a better idea of what sex it is. If the snake is very small and/or young, the margin of error also goes up a bit.

I did visually sex two WC newborn concinnus' back in the late 80's and raised them up to a ripe old age. I wanted a pair when I collected them, and I wanted babies. They were with me a long time. (male 18yrs, 3ft, female 21 yrs, 3.5 ft). Apparently I got it right. In their lifetime with me, they produced litters and I released 150 yearlings back where I found the pair.