One pixel each for all 16777216 colors — no repeats allowed
RGB color space is a way of defining colors as represented by the chromaticity of three colors: red, green, and blue. By adding together varying amounts of each of the three colors, you can more than 16 million colors, closely matching the limits of human vision. But what if you tried to make an image using every single possible value of RGB? With one pixel for each color, and no repeats allowed, you have the allRGB project.
allRGB is a repository of almost 100 images, all of which follow those same restrictions:
one pixel for every RGB color (16777216); not one color missing, and not one color twice.
What's interesting is how varied the approaches are to achieving this goal. Some are just simple patterns, and can clock in at just a few kilobytes in size. Some, like ChillyRun2, mimic real photos, and is more than 50 megabytes large.
If you're interested in trying it yourself, there's a GitHub repository for generating the images yourself.