Portraits like this shot by Andrew Semansco, used under a creative commons attribution license, are perfect for the Warhol Pop Art effect, but you can do this with any picture you like. Try following this tutorial and then have a go with one of your own photos.
Step 1 Duplicate the original layer and create a third layer between the duplicate and the original background.
Step 2 Fill the empty layer with white and then delete the original background layer.
Step 3 Now we want to separate our model from the background of the image, the best way to do this with the cleanest edge is to use the Path Tool to draw a path around her and then cut away the areas we no longer require. Select the Path Tool and draw an outline around your model, it’s best to read the GIMP user manual for this tool here if you’ve never used it before. You can drag when you click to warp the line to fit curves such as her shoulder more closely, and you can move the individual points by dragging them once they’ve been placed. When you’ve gone all the way around the model and along the edges of the image you can hold control and click the first point to close off your selection. Your finished path will be saved in the Paths pane, where you can right click it and choose ‘Path to Selection’. Invert your selection with Select>Invert and press delete to reveal your white background layer.
Step 4 We want to make our image greyscale before we can begin this process, so open Colours>Hue-Saturation and bring the saturation slider to zero.
Step 5 Now open the Colours>Brightness and Contrast menu to increase the contrast and play with the brightness, we want a fair amount of black and white in the image so play around until you get something with a lot of impact.
Step 6 Then open the Colour>Levels menu and move the left-hand arrow across a bit to boost the dark tones in the image.
Step 7 Now we have to think about the process through which we can create that Warhol effect. Warhol used silk screening, a process where each layer of detail is printed on top of the previous layers, working from the background to the fine detail. To do this while allowing ourselves the greatest amount of control over colour we would be best to separate each level of detail out into a layer of its own, recreating the silk screening method, whilst limiting the number of colours used. To begin this process we will use the Gaussian Blur tool to blend the colours slightly, use a value around 4 or 5. Then we want to use the Posterise tool to set the number of colours in the image to only a handful, I’m using 4 so that I get a black layer, a dark grey, a lighter grey and white.
Step 8 To set up the numerous layers of colour for our screen print inspired method we shall first make up a few new layers. We need one for the black and two greys, one for her skin and another for clothes/makeup etc. Create and rename these layers as appropriate, and then order them Background>Skin>Light Greys>Dark Greys>Black>Clothes etc.
Step 8 Now use the Colour Selector tool to copy each colour to their own layers, anchoring each pasted item to their layers. Use the Magic Wand to just select the white areas of the face to paste them into the ‘Skin’ layer then use the same tool to select the clothes in the picture and paste them into their layer too.
Step 9 Fill the White Layer with a colour of your choice, then enter each layer in turn and use the Colour Selector to select the areas you pasted before once again. Using whatever method you prefer, I used the Paintbrush, colour them in; only the selected areas will be coloured.
Step 10 Duplicate the Clothes Layer and place the new one at the top of the list, then change the layer mode to multiply. Now you can manually add makeup, if you want to, and change the colour of her clothes and eyes.
Step 11 Now that we have finished colouring in our picture, we can finish off this project by flattening the image and then applying the Filter>Artistic>Apply Canvas to recreate the texture that is present in Warhol’s original pieces. It is only slightly visible in Warhol’s prints though, so a value of 2 or 3 should be sufficient, otherwise it will look too fussy and noisey.
If you like you can open a new project in GIMP that is several times the size of the original image; for example a project twice as wide and twice as high would mean there would be four repetitions of the image, and tile the image. To change the colours in each version use ctrl z repeatedly to return to step 5 in your original project and fill each area with different shades, flatten these again and re-apply the canvas before pasting them into your new project and arranging them as you wish.