It's not mentioned, but this function removes all profiles, etcetera from the image.
(PECL gmagick >= Unknown)
Gmagick::thumbnailimage — Changes the size of an image
Changes the size of an image to the given dimensions and removes any associated profiles.
The goal is to produce small low cost thumbnail images suited for display on the Web.
If true is given as a third parameter then columns and rows parameters are used as maximums
for each side. Both sides will be scaled down until the match or are smaller than the parameter given for the side.
widthImage width.
heightImage height.
The Gmagick object.
Throws an GmagickException on error.