Trigger Warnings

More of a question than a debate. How exactly are they supposed to work? People on her seem to say trigger warning and that's it. Are people who need trigger warning expected to just not go near anything with a trigger warning even though it may trigger them? I'm just asking because of the baby spider picture, I didn't have an anxiety attack or anything but I would have much rather not seen that (not good with spiders at all) so that got me wondering.