As I work regularly with CNC machines (these are milling machines that can be controlled by a computer), I want to show you two methods that are being used to engrave images in a pixelated style which you can feel with your fingers.

The first method is used to make half tone images, which is very similar to low resolution images in newspapers. The difference is that the dots are milled using a V-bit, so you can feel every dot. An example is the following image (made by metalfusion):

Another method is used to make lithophanes. These are images made of a thin material that is translucent (Corian is often used), engraved with an image. Dark parts of the image are made by keeping the material thicker, as light parts are cut away to make the material more translucent.
Using this method you get the high-contrast lines as extra difference in height of the material, while low contrast parts are only a small difference in height. An example of a lithopane milled in corian is this one (found on the website of Vectric: software that can make lithophanes)

In the first image you can clearly see the high contrast lines being a good height difference in the material.

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To answer the questions in the comment:

Lithophanes are made with a ball-nose. You could use a ballnose of say 1mm. That would technically give you a pixel (dot) size of 1mm. However, the CNC machine has a resolution of say 0.01mm (at least) and can change it height every resolution step. So in practice you get high resolution but your contrast is limited to the size of your tool.
In the iamges below you'll see the effect of the large toolsize compared to the resolution.

About the colors, I've never seen a multi colored lithopane. However you can use colored material as in the images above. It is probably possible to print over a lithophane, however the backlighting will not work so you can just as easy make a relief (such as the maps with terrain height relief).