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I was looking at what's inside C:/Windows/System32/imageres.dll (which contains most icons used by windows 8.1) and noticed it also contains some files inside its RCDATA which are not image files.
After opening those files in Notepad++, I noticed that they all seem to start with the magic number "CF AC 07" followed by some kind of header. After that, a series of png images follow (around 5-10 per file). I can then split the file into several png files and open them with an image viewer.
Here is a zip file containing all the split images: https://github.com/Runemoro/files/blob/master/png.zip?raw=true
And here's an imgur link with a few of the images: http://imgur.com/a/3PnIo
The png images in each collection seem to be related. For example, some of them stitchable together to make a larger image.
Some of the files (20400-2.png, for example) have metadata that tells us that they were made with "Adobe Photoshop CS6 (Windows)" in 2012. Tineye doesn't give any results when searching for the image.
What are these images used for, and what is the format of their containing files?
1Seeing how this is a popular question, if anybody who sees this has a better version of the image in this answer, it'd be nice if you edited that in. – user1306322 – 2016-09-18T17:08:30.463