ImageResize[image, width]ImageResize[image, {width, height}]The Resampling option can be used to specify how to resample the image. Options are:
See Pillow Filters for a description of these.
alice = Import["ExampleData/MadTeaParty.gif"]
shape = ImageDimensions[alice]
ImageResize[alice, shape / 2]
The default sampling method is “Bicubic” which has pretty good upscaling and downscaling quality. However “Box” is the fastest:
ImageResize[alice, shape / 2, Resampling -> "Box"]