@ssp3 You can edit the content of posts (attachments or otherwise) by requesting it to admins/mods and providing them with what will be the replacement. In your case, you need to create the specific watermarked pictures you want to replace the original pictures with, and appropriately send them over. Then you need to specify what posts they will go to, and provide their URLs.
I think this is fair of you to request, especially if the content is sensitive (which doesn't seem to be the case here, but that's besides the point), as long as it doesn't become common practice: from now on you should always upload them watermarked, so that the very limited forum resources (admin/mod time) are not irresponsibly wasted.
But to fulfill what you are asking, there is absolutely zero need to compromise the forum engine as a whole: We shouldn't systematically allow this for an edge case, at the risk of facilitating the destruction of history, as @Knezzen correctly alluded to.
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On a personal note, I get concerned seeing the sudden switch from the casual, amicable language to "Since there's nothing in Registration Agreement about transfer of ownership of original content to forum owner, from legal point of view [...]", because we are not arguing in those terms, and ideally we all should be discussing things in good faith. But this approach and language is destructive, and I strongly condemn this: it shifts our focus from the theme and goals of this forum, which is OS9, to, of all things, legal technicality and jargon. It raises distrust, and makes us defensive. This is not worth it, and anything that shifts us to that direction is a threat. People here are generally good, amicable, and will listen to your plea if there's anything you want edited within reason, without any need for such approach or language.
Incidentally, even if we are to go down that route, the Terms of Use ("Terms and Policy" as it is called on the bottom right corner) can nonetheless be accordingly updated with immediate effect to reflect this, perhaps with something along the lines of "While you own what you post, the proof of which being the fact that you can freely repost and reuse your content anywhere else, posts that are older than X and/or were responded to freeze and can no longer be edited within these forums, and the staff is not responsible nor has the obligation to spend its limited time resources to make such 'post-mortem' changes for you on a whimsy. At most, you can contact staff nonetheless, but the staff won't address the matter unless if the content in question is extremely sensitive, and that too only according to the staff's own discretion.". (I hate the fact I even "had" to type this sh!7: It shouldn't have to come down to this, I'd rather have used that time for something OS9-centric. Also of note: I'm not part of the staff, nor do I aspire to be.)
TL;DR: C'mon. Let's remain reasonable. We don't mess the forum, but if you want to retroactively watermark your stuff that badly, simply send them over, along with the links of the posts they correspond to. We might be able to go from there instead.
