New answers tagged pixel-art
You can try the standalone editor at http://juliandescottes.github.io/piskel You can use Piskel to do two things : spriting : create retro-style sprites for games pixelart : create crazy/pretty pixelart animations for fun ! Offline version: Offline builds are available. More details in the dedicated wiki page.
The first part is realy easy. Don't scale your image using any interpolation method. Choose None in the dialog box. Use exact multipliers.
You can create layers with 100x100 pixel squares on a 800x800 pixels document, move them around and changing its colors.
The answer seems to easy too me so I guess I did not understood it well :) But did you try: Filter > Pixel > Mosaic?
The easiest way is to change the image to Indexed Color instead of RGB via Image > Mode. All transparent pixels will be removed and none will be created when the image is downsized. Alternatively, you can duplicate and merge the downsized layer repeatedly. Even a pixel with 1/256th opacity will be opaque after 8 iterations.
Unless your shape only contains 90º angles, it will have semi-transparent edges. The reason is that pixels are square, so Photoshop tries to compensate for half-pixels with transparency. I made a little diagram to show what I mean by half-pixels:
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