Manual:$wgJpegPixelFormat

Details
At default setting of 'yuv420', JPEG thumbnails will use 4:2:0 chroma subsampling to reduce file size, at the cost of possible color fringing at sharp edges. This is usually not noticeable with photographic images, but can cause visible artifacts at sharp red and blue edges.

Supported values:
 * false - use scaling system's default (same as pre-1.27 behavior)
 * 'yuv444' - luma and chroma at same resolution
 * 'yuv422' - chroma at 1/2 resolution horizontally, full vertically
 * 'yuv420' - chroma at 1/2 resolution in both dimensions

This setting is currently supported only for the ImageMagick backend; others may default to 4:2:0 or 4:4:4 or maintaining the source file's sampling in the thumbnail.

Related settings
Currently only supported alongside Manual:$wgUseImageMagick. Other thumbnailing backends will use whatever their default is.