This .zip contains a "ledsign" folder. Extract inside your /materials directory. Inside are two .vtfs and a .vmt as well as a .bsp that displays the example materials.
Feel free to modify the materials and textures and whatnot. leddetail.vtf is just a radial gradient and you could probably make something nicer if you spent more than 5 minutes on it. Credit me if you use any of this, but I won't ask for money.
Credit to imgur user MundaneMullane for the sample image (sign01.vtf). If for whatever reason you want to use the sample image in your project, ask them for permission first.
HOW TO MAKE YOUR OWN:
1) Find the image you want to use for your sign. Chances are, your image is rectangular. The technique involved to get the pixel effect on the final material requires the base image to be square, so add blank space to your image so that the final image is square.
2) Make a VTF of your image. If you want the image to appear very crisp, enable the "Point Sampling" flag on the VTF. This will make the individual pixels on your image have hard edges between them instead of blending them together. This makes it appear more accurate up close, but it can also cause visual artifacts when the player is further away (notice the "screen door" effect on the first screenshot). Pick your poison.
3) Take the provided .vmt and copy/modify it to use your base texture. In addition, change "$detailscale" to whatever the size of your base image is. This tells Source how many times to repeat the LED image over the entire texture. If your image is 512x512, change this to 512.