I hate spiders myself, so I completely understand where you're coming from here, but unfortunately we can't make it a rule that trigger warnings must be included with certain links posted.
The reason for this is that there is no limit to things that can be triggers, and it would just get really messy and nearly impossible to manage. Spiders are pretty common to be bothersome, but it isn't unheard of to be disturbed by images of cute kittens or puppies either.
If you know that certain things are bothersome or are triggers for you, the best solution is to not click the link blindly. Ask the person for clarification of what the link is. Pay attention to the words in the link's URL. If they don't respond and no one else responds, don't click it. Chances are more often than not you're not gonna be missing anything anyway. We have to assume some personal responsibility here for what we choose to click, and not just leave it up to the other party to make sure it is "safe".
Now, if someone was posting links of spider pictures and disguising it as an advertisement, or flat out lied to you when you asked what the link was, that would be intentionally misleading, and we could definitely take action there because it wouldn't be okay.