Use the left and right arrow keys to select and image, then press space to start scanning the image. The program will divide the image into 5px by 5px chunks and take one color from each chunk. Then it will reproduce the image. To scan in your own image, simply add a backdrop with your image and select it.
It takes a long time. That is normal. If it only draws a small part of the image, try reloading the page. Credit to @griffpatch for the square-drawing code, and credit to @DadOfMrLog for the color detection code. Version history: 0.1 Built 0.2 Added image reconstruction 0.3 Made pixels square 0.4 Improved resolution 0.5 Added more backdrops 0.6 Added selection tools