The infographic should actually be called "Pit Vipers vs. All Other Kinds of Snakes" and there are lots of potentially dangerous snakes that are not Pit Vipers.
Let's get one thing out of the way: the snakes that are potential health hazards are not poisonous, they are venomous. Here is a fun comic from Bird and Moon to help you remember.
As hinted in the comic, there are a couple exceptions to this rule.
But enough semantics. Pit vipers do have heat-sensing pits (hence their name) but there is another group of venomous snakes, the elapids, that do not. The Elapidae family includes coral snakes, sea snakes, cobras, and those famous Australian characters like the Brown Snake and Taipan. Elapids generally have a neurotoxic venom (it affects the nervous system) as opposed to pit vipers, which generally have hemotoxic venom (it affects the blood). These are not snakes you want to mess with.
Elapids also have round pupils and two rows of scales underneath the tail. So, as you can see, if you use the above graphic to distinguish between harmless and potentially dangerous snakes, you may be in for a nasty surprise.
Please share a link to this blog post wherever you see the above graphic being shared - you might save someone's life!
Update: An astute commenter has pointed out my North American snake bias (below): "You should perhaps point out that it isn't ONLY pit vipers that have heat pits. Many harmless pythons and boas have them too (though they have more than two). And the shape of the pupil is in no way diagnostic - many snakes have a slit pupil in bright light which expands to a circle in dim light."