The Wondrous Pixel Turtle is a filtering tool to apply a pixel art style on images. It might be a bit slow, especially when handling large files, but generates good results.
Note: MSE might issue a warning about the exe. This is a false positive. Either tell MSE to allow it, get another antivirus, or switch to Linux. Compile it from source if you still feel insecure.
Most images on this page are Creative Commons. See here for further information.
