Since a security update of ImageMagick TYPO3 generates images which are really dark. The problem seems to be the colorspace setting. ImageMagick changed the default behaviour. Changing or adding sRGB as a value to the colorspace key in
/typo3conf/LocalConfiguration.php fixes this issue!
'GFX' => array( (...) 'colorspace' => 'sRGB', (...) )
While you’re at it, make sure you have your jpg quality settings right… I find 80 to be an optimal value for the jpg_quality setting resulting in relatively crisp images while keeping the filesize low:
'GFX' => array( (...) 'jpg_quality' => '80', (...) )