How to make image maps on other servers in Minecraft?

Consider this picture:

I used an image to map converting tool which I found somewhere on GitHub.

The thing is, I saw an item just like this on a public Minecraft server. Somebody there offered to sell it to me. How did he/she do this on a server? He wasn't an admin or anything.

The only way I know of creating these 'image maps' are by modifying the map.dat files, which requires administrative privileges over the server/world.

You can go through the painstaking work to place the appropriate blocks on a horizontal plane to make the maps.

Here I have placed down the blocks to make my name: