I don't see anything. Is it just me...?

I'm not seeing it either. But if you download it, you see blue stripped background.

It's a reference to her pantsu

You open it in IE, THEN, you select the image WHILE it's maximized.

I think I understand.

There are very small pixels arranged in barely visible stripes in the background.
When it's a thumbnail, a bunch of pixels are removed to make the image smaller.
The pixels are probably arranged with the knowlege of which ones are removed when creating the scaled-down image.
so- basically all the white in between them is removed, while the black(blue?) remains.

Am I right?
Or is it just so obvious noone's bothered saying it?

Don't know if this is the typical ie select all trick. However, based on what you describe I'll give you a hint. ...

Damn, when I read 'if you download it,' and 'very small ... barely visible'- I was getting all hot and bothered about tracking down another person using a different trick that I've been using that works even with many other non-ie browsers. But after quick inspecting it, it's just a typical transparency-highlight trick. (which is still clever but not what I was hoping for based on your assumptions)

Given you said '(blue?)' I assume you might not have yet seen the effect of highlighting the full image in IE (after clicking on the direct link and hiting ctrl-a)? It should be obviously blue.

The pixels are probably arranged with the knowlege of which ones are removed when creating the scaled-down image.
so- basically all the white in between them is removed, while the black(blue?) remains.

Am I right?
Or is it just so obvious noone's bothered saying it?

Nah, this case is less complicated. At first that sounds like you are partly describing a cross between the ie_specific_highlight_trick described here: open_in_internet_explorer, and another trick. And then later, a description of taking advantage of interpolation. You are close in some respects, but not quite there, and most of that doesn't apply here. Rather then explaining all those cases...

Lazy Explanation: Your 'barely visible stripes' are solid areas of transparent pixels that turn blue when highlighted. The black stripes in the thumbnail indicate where these transparent areas are in the original. (It's NOT exploiting the grid masking described on the wiki. ie. In this case it's not hiding or revealing an image, only coloring it.)

Why the black stripes in the thumbnail? Well, in the original image those areas are solid stripes of transparent pixels. Only a few image formats support transparency, and all the db thumbnails are JPG, which do not. Any transparency is black in the thumbnails because ImageMagick (or whatever tool being used to create thumbnails) has ended up replacing transparent pixels with black when converting it to a final JPG thumbnail. Note the black backgrounds in these, then click one: transparent_background

Overall Effect? You'll notice that the black areas in the danbooru thumbnail turn white (see through) when the full image is viewed, and that the areas between the black-stripes are still white, which makes it look like a completly white background. --- Then, when the full image is highlighted, the transparent-stripes are colored dark blue, while the white-stripes stay white but get a thin blue-checkered pattern on them. The checkers on the white-stripes are a result of the ie_highlighting and have nothing to do with the original image. (When viewed in explorer on your desktop or in a folder it may act similarly. Gnome users will notice an opposite effect when you select the saved file and it's thumbnail is highlighted, because transparency IS supported.)

(when using ImageMagick, if resizing with transparency you can turn it to white by using the -background white -flatten options, otherwise you often get the black patches)