Everything that is output to your computer screen is made up of tiny little squares called "pixels." Each pixel has a single color, and the mixture of all these pixels together makes up the colors that you see. Usually, pixels are too small to see. However, if you've ever zoomed in on an image close enough, you know that pixels exist. Pixel art is a minimization technique whereby each pixel of a picture is made big enough to be clearly visible. Using Photoshop, you can make any image into pixel art.
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Open Photoshop. If you are on a PC, you can do this by going to "Start," "Programs," "Photoshop." If you are on a Mac, go to your Applications folder and click "Photoshop."
Click "File" and "Open" and select the image that you'd like to make into pixel art.
Go to "File" and "Save as" and save your picture with a new name. This is so you don't lose your old picture.
Go to "Filter," "Pixelate" and "Mosaic." The mosaic pixelate function is interactive, so play with changing the cell size of the function to make your image more or less pixelated. When you're done, click "OK" and then save your new piece of pixel art.