A quick and easy Warhol effect
1. Take your original image and resize if necessary.
2. Convert to greyscale using any method of your choice (but make sure that you end up with a genuine greyscale image i.e. Image -> Mode -> Grayscale).
3. Adjust the Levels -- to taste but you'll probably want to over-adjust to get slightly or completely blown-out highlights and nice dark shadows. Depending on your image you may also want to use different levels adjustments for different parts of the image -- here I have used one levels adjustment on the sweater to make it darker (as I didn't want it to fade away to nothing) and another adjustment on the rest of the image.
4. Create a new image (File -> New) with the same dimensions as your resized original. Fill the background layer of the image with a solid bright color.
5. Add a new layer and fill with a contrasting solid bright color.
6. Still with the new layer (Layer 1) activated, do Select -> Load Selection (Document: your resized source image; Channel: Gray).
7. Now do Layer -> Add Layer Mask -> Reveal Selection.
8. Duplicate this image as many times as required for the size of the finished piece you have in mind. Here I shall create a 2x3 grid of images so I need this version of the image plus five duplicates.
9. For each duplicated image, fill the Background layer and Layer 1 with contrasting colors.
10. Once you have all the images completed to your satisfaction, flatten each of them.
11. Create a new document (File -> New) with the dimensions of the finished piece. In this example it needs to be twice as wide and three times as high as my resized source image.
12. Drag each of the images onto the new document as a new layer and arrange them to suit