I definitely think content warnings (not specifically “NSFW”) are ideal for this. Sure, the line between what should and shouldn’t be content-warned is blurry, and you don’t need to go through every highly-specific issue someone might have with your post, but things like tripophobia (or arachnophobia) are pretty widely known so it makes sense to warn about it imo. Especially for image posts since you can’t really just “not look at it” if it pops up in your feed.

And whether or not the warning is precisely about a “phobia” isn’t relevant imo. It’s just a nice thing to do, as it avoids causing others to feel suddenly itchy and uncomfortable for several minutes.

So no, I wouldn’t tag spider images as NSFW, but I’d definitely add a “CW: spider” if Lemmy supported it 🙂