Cropping
is a method of removing some of the original file that is not needed.
use a graphics editor to include only the parts you want in the image.
Resampling
Optimisation of files for digital print or online by changing the number of pixels in the image using a graphic editor like Photoshop
make sure you work on a copy not the original
always use maintain aspect ratio,
if resampling for screen use, change the resolution to 72 dpi
if resampling for print use, change the resolution to 300 dpi
tick the Resample Image box before you click Ok
Follow the tutorial here