Three tags, all with extremely close and inconsistent use. It's a mess.
Shocked and surprise, aside from being plagued with barely-related posts, are both being used for almost entirely the same situations, except that surprise also has a smattering of "surprising event" posts (e.g. post #4599548
It doesn't help things that surprise's wiki says that "If a person experiences a very powerful or long lasting surprise it may be considered shock(ed)", because we all know how well tagging goes for temporal themes.
surprised_expression is a little more consistent because it doesn't have a competing tag and specifies that it's for a visible expression, but I think its name has a tendency to cause people to pair it with surprise automatically. Which would explain why so many posts in surprise look like anything but.

So:
What's the real difference between shocked and surprise? Do we need both?
If one is supposed to be an event and the other is supposed to be a state of being, one of them is mostly redundant with surprised_expression, right?

shocked has the added ambiguity of being mistaken for an electical shock, rather than a shocked expression (one can cause the other, but not always). This is way I'm a big proponent of standardizing on *_expression tags for things like this. And I'm not sure there's much of a difference between shocked expression and a surprised expression, at least not a meaningful one when it comes to images.

Oh, and there's also startled, which has fewer uses but is almost certainly redundant with the topic tags.

watsit said:
shocked has the added ambiguity of being mistaken for an electical shock, rather than a shocked expression (one can cause the other, but not always).

https://youtu.be/LM4f_Ghx-o0?t=65

There's a BUR and more discussion over at topic #42177

donteven said:
Oh, and there's also startled, which has fewer uses but is almost certainly redundant with the topic tags.

I always associated startled with more of a fear response. Like the shock or surprise a character experiences is out of fear and not happiness, harmless unexpectedness, etc., so it would be tagged as startled specifically.