WHOO okay that should be everything. To recap what I learned hecking up and then fixing this thread seventy times:
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Dropbox is good for linking pngs for transparency, but since the forum doesn’t convert the images and they remain hosted by Dropbox, you’re subject to Dropbox’s bandwidth limit. Every single preview image and link will break if you hit that.
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Linking from imgur (which apparently does not enforce a bandwidth limit) causes the forum to re-upload the image as a smaller jpg if it is over 2mb. Imgur’s [img] tags do not prevent this.
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Aforementioned tags DO force image urls to stay images when you use the blurring [spoiler] tags. Without the [img] tags, the spoiler tags will turn an image link to a regular link. (This should probably also work for the dropdown/summary spoiler tags. I am not sure if this is possible for gifs, it didn’t work on mine.)
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Blurring an image breaks the ability to view the full image (via both “new tab” and “view image”) if the original is larger than what the forum shows. A hyperlink around the entire thing causes the click-to-unblur function to open the link. Putting the hyperlink inside the spoiler tags makes it possible to click blank space on the right (if applicable) to unblur, and the “times viewed” circle to become right-click-able, but is not an intuitive method. Placing a fullsize link above or below the image seems to be the best option.
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If an image is above 3mb, it will be converted to a link regardless of what you do. Imgur allows for resizing, so fixing this is relatively painless. (And it functions by adding “?[EditNumber]” after “.png”, leaving the original accessible, so you might be able to fudge linking the original with the addition removed. However, I am uncertain how long/if imgur saves older edits, so reuploading a larger version to link might be safest.)
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File size appears irrelevant if you have transparency in your image (plantergeist is originally 540kb). It will be converted to jpg and you will be sad.
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Changing the image dimensions to what they would be as a thumbnail maintains png transparency. 690x500 appears to be the maximum width and height, respectively. Resize via imgur if necessary, and if you want to give access to a larger version, hotlink the image (or put the link above or below, if you use spoiler tags).
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There is a maximum number of edits per day. If you hit it, you have to wait 3 hours before you can make more.
All that said, NOW the thread should be good. Everything should be viewable, and if the image previews break again, I will probably just crawl under my chair and sleep for a week instead.
Some things were wallpaper dimensions and no longer are, for the sake of file size. If you want want one, try removing the “?[EditNumber]” after “.png”, and if that doesn’t work just let me know and I’ll fetch you a proper link. 
EDIT: New trick learned:
- An image (under 3mb) directly uploaded to the forum
and then placed inside a dropdown spoilercan break the thumbnail rules. If very long (presumably at least twice as long as it is wide, whether inside a spoiler tag or not), it may size itself to fill the width of the post, resulting in only the top section of the image visible. Example:
