Full-colour images

Computer screens can now typically display 256 levels of intensity in each of the three channels - red, green and blue. This means there are 16,777,216 - about 16.8 million colours available.
Imagine a bitmap image 4096 pixels wide by 4096 tall[1]. This has the same number of pixels as there are colours available.

It's straightforward to procedurally generate images which simply display every colour, of course.
But the idea here is to create interesting, artistic or attractive images which use every single colour value available. A dedicated drawing program could help with this, providing useful features to make sure there were no repeated colours, using dithering to eek out more of a desired hue overall in an area, graduated fills and so on.

Obviously, one could do similar at a smaller scale, making images at a lower bit depth. Five bits per channel for a total of 32,768 colours is a fairly common format - images with a resolution of 256 by 128 pixels could use all of those, and similarly, 64x64 images could display every colour of 4-bit per channel (12-bit/pixel) images.
And 16 by 16 images could use every greyscale value.

[1] Or alternatively, 8192 by 2048 landscape or portrait, etc.