We will be guided in our pin placement by a low resolution indexed color gif image we will create in Photoshop. The .gif will use a limited palette restricted only to those colors in which the plastic push pins come.
This tutorial assumes you have some access to and basic understanding of Adobe Photoshop.
Step 1: Select a photograph
I'm using a photo I took of a woman who sells her hats and headdresses on the internet.
Step 2: Decide the final dimensions of your work in inches
Step 3: Convert the photo to a .gif of limited colors
Step 4: Count pixels of each color so you can buy pins accordingly
Using your magic wand tool set to non-contiguous regions, tolerance = 0, visit your tiny 90 x 120 .gif and select each color in turn.
Record the count reported by the histogram for each color, That's ho wmany push pins of that color you'll need.