Manual:$wgUseImageMagick

Details
Resizing can be done using PHP's internal image libraries or using ImageMagick or another third-party converter, e.g. GraphicMagick. These latter supports more file formats than PHP, which only supports PNG, GIF, JPG, XBM and WBMP. Set $wgUseImageMagick to true to use ImageMagick instead of the builtin functions.