Cropping cuts away part of the image - it changes what is shown. Resizing keeps the whole image but changes its pixel dimensions - it changes how big it is. Most tasks need crop first (frame the subject), then resize (hit the required size).
Cropping
Open the Crop Image tool and upload your photo
Drag the frame corners to enclose the area to keep
Apply and download - everything outside the frame is discarded
Resizing
Open Resize Image and upload the file
Enter the target width or height - aspect ratio is kept automatically
For a required ratio (like a square avatar or 4:6 passport photo), crop to the right proportions first, then resize
Download the resized image
Common requirements
Passport/ID photos: usually a strict ratio - crop first
Website uploads: often a max width like 1200px or 1920px
Email attachments: smaller dimensions also reduce file size
Never enlarge a small photo for print - it will blur