Before you go for your first black-and-white portrait with huge enthusiasm, you should be aware that planning such a project can take a very long time.
The first thing you need to do is sort your dominoes by their shade of grey. You can do that by watching them through your camera on black-and-white mode. And the next step is important: Take two photos of the dominoes, one in color and one in black-and-white. Liek that, you can later assign the grey shades to the colors. Once you've done that, you should look for a portrait to build. There are practically no borders here, but best suitable is a frontal picture of a person (or an object).
Keep in mind how detailed the picture is that you're trying to plan. If it is a very detailed picture, a lot of dominoes are required in order to get a decent picture. I recommend to do this: Look for the tiniest detail in the picture and make that detail just so big that it will be represented by one domino. That makes sure that the resolution is high enough for the picture not to look crappy. For a great portrait, you need at least a 5-digit-number of dominoes and a lot of space. Of course, there's no theoretical limit. My biggest portrait was a labrador dog with 12,240 dominoes.
So you've found your picture - now the planning can start. There are several ways to do that; the best works with a freeware called GIMP. Open your picture with it and, if it isn't black-and-white yet, make it so by setting the "color saturation" to 0.
Now you need the gray scales again. Using the photos you made of your dominoes and their gray scales, replace every gray scale in the picture (after changing its settings to including only a few different gray scales) with the color that belongs to it. Luckily, GIMP will do that for every pixel of a certain grey shade with one click. Once you've replaced every grey scale with its color, your field plan is finished.
If all of that went well, the result is a very nice field. The technique is one of the most laborous ones, but in the end it's worth it...