Wikimedia Bugzilla is closed!

Certain, but not all PNG thumbnails can be discolored, as seen here. 

This image:
http://wikidiy.com/mediawiki-1.9.1/index.php/Image:Looking_over_paper_657.png

is RGB, true color, 24 bits

This image:

http://wikidiy.com/mediawiki-1.9.1/index.php/Image:Red_copyright.png

is RGB, indexed color (127 colors), 24 bits. 

Forcing imagecreatetruecolor in Image.php seems to fix the first thumbnail:

     if ( true ) {
          $dst_image = imagecreatetruecolor( $width, $height );

but doesn't work for the second image, possibly because it has a transparent
background. 

I'm unsure if this is related to bugs 6205, 5994 or 234, but is reproducible on
our installation with libpng 1.2.16 and gd 2.0.34.