Is there a rule against using filtered images?

No,* although I was referring to why the image as-is wouldn’t be used. Images sometimes undergo minor editing for things like color correction, watermark removal, etc. It’d be preferable if the original image didn’t have the red eye, but the correction isn’t a huge deal. The poor lighting is the much more severe issue.

* There are different levels of “rules” on the English Wikipedia. I’d categorize them into “policies with legal considerations”, policies, guidelines, the Manual of Style, and norms.

• Policies are widely accepted Wikipedia standards everyone has to abide by like “verifiability”; the ones with legal considerations are even more serious like “libel”.
• Guidelines, like “offensive material”, are best practices supported by consensus that editors weigh when making decisions. Often more specific than policies.
• The Manual of Style does what it says on the tin and answers your question about red-eye correction. It’s concerned with nitty-gritty technical stuff like when to use certain punctuation, how long a lead section should be, etc. Everything everywhere must abide with few exceptions, although the MoS is so extensive that things slip through the cracks all the time – usually inconsequentially.
• Norms are informal standards outside of policies and guidelines that editors (sometimes only in a specific subject field) usually agree on. As a specific example, most major cities of the world have a collage showing different landmarks, but this isn’t written anywhere. Wikiprojects (collaborations over a specific field, e.g. astronomy) often have their own best practices for their specific fields. And there are “essays” – which are opinion pieces editors can easily link to that often describe norms (they don’t have to; they can just be an editor’s pet peeve) but which aren’t binding. A classic example of the essay is about “coatrack articles”.

At random:

• We maintain a nuclear bomb of a party trick for nerds, which is a list of unusual Wikipedia articles.
• We have our own newspaper.
• If you go to “View History” on an article, there’s a link called "Page Statistics that does what it says.
• We keep a list of perennially discussed sources which can often be a decent litmus test for source quality.
• We explicitly have stricter guidelines for sourcing in medical articles.
• There’s a specific guideline about living people notable only for one event.
• Many single-sentence species articles you see were generated years ago by a bot. Any formally described eukaryotic species is automatically considered notable enough for its own article.
• Wikipedia has portals which act as landing pages for different subjects.
• Administrators aren’t glamorized or self-important like is often aasumed outside Wikipedia. They have a mop badge, are voted in, have strict codes of conduct,, and only number about 800. When people complain about admin abuse on Wikipedia, 99% of the time in my experience it’s sounded like a regular user (who, with few exceptions, has every right to do this) didn’t like their edit and reverted it – but they assumed that was an administrative act.
• Wikipedia has varying levels of protection for articles. Extended protection, requiring an account with 500+ edits and 30+ days old, is the highest one you’ll normally see for highly controversial topics like Donald Trump, ongoing international conflicts, etc. The exception to this is the Burger King Whopper, which was extended protected for nearly a decade after Burger King tried to abuse Wikipedia for a commercial and caused a tsunami of vandalism.
• There are awards.
• I think everyone should try Wikipedia’s sister projects at least once. The English Wiktionary, for example, is a fantastic everything-to-English dictionary.
• A huge chunk of our species images comes from iNaturalist.
• Thanks largely to one insane editor, Wikipedia is the greatest single resource on New York City architecture ever created, and I would bet my life to a pack of chewing gum on that.
• The “Did You Know?” list on the front page isn’t just random facts. An article can only be nominated to be there seven days after it’s created, expanded by 5x, or promoted to Good Article status (99% of the time it’s the second or third). So usually, clicking on the article, you’ll find a niche subject that someone very recently put a ton of passion into researching and writing.
• There’s a beginner-friendly Q&A forum called the Teahouse.
• Most media used on Wikipedia is hosted on our sister project, Wikimedia Commons, not Wikipedia itself. Exceptions are usually fair use material.
• There are new page patrollers who can clear new articles for indexing by search engines.
• There are three types of article deletion: speedy, proposed, and articles for deletion (AfD).
  • Some articles meet very narrow criteria and should be deleted as soon as possible without discussion, in which case an editor can nominate them to be checked by an admin.
  • A proposed deletion, or PROD, happens when an editor tags an article as such and leaves an explanation. Anyone can remove the tag for any reason, and a PROD can never be placed on that article again. If the tag remains up after seven days, an admin will read the PROD and decide if it’s valid.
  • Articles for deletion (AfD) is what you’d probably think of. Someone nominates an article for deletion, and for at least seven days (often more if there’s no consensus, and rarely less per a snowball clause), the article is discussed – usually on the grounds of notability. It’s not a vote, but people do summarize their argument as “Keep”, “Delete”, “Merge”, “Redirect”, etc. The discussion is reviewed by an administrator who has (basically) final say and has to decide on the most compelling argument in terms of guidelines and policies – although it’s very rare for them to noticeably break with the overall discussion.
  • Articles deleted by PROD or speedy deletion can be readily recreated, while those discussed at AfD cannot – those need to overcome the discussed concern.