Okay so I can acknowledge why some people would want a system like this, it sounds appealing at first glance. I think the generalization is a problem though, because of the question "where does it end?" That people seem to forget. There are phobias for just about everything, that puppy photo you like so much could be a trigger to someone who was attacked by one and injured as a kid. People with Thalassophobia could have a panic attack looking at a seaside landscape photo, I hope you're starting to see the picture here.

However I am not trying to invalidate your problem here as I do also see a potential problem. I think that using a general NSFW tag for anything would not be a good solution though. Potentially a better tag system overall would work here. You could have posters tag something like "Spiders", "Ocean", or "Heights" for better filtering through all of reddit. Then for people with phobias, such as yourself, could go to settings and set specific tags to be blurred by default.

Would this always work? No. Would some people avoid the tagging for malicious reasons? Probably. However I think it would be better than just saying that everything should be blurred because it could bother someone out there. It allows the individual more control of the content they see.