The source file is tamarin-pixelart-16.kra which was created using
krita.
This can be used to export a 16x16 png file, which can be upscaled (no smoothing) to the 32x32 and 64x64 versions.
The source file was also used as the basis for
tamarin-pixelart-large_grid.xcf and
tamarin-pixelart-large_grid_details.xcf. Both files are 420x420
pixels, and are intended for the gimp editor. The second file has
more subtle shading and highlights.