Go to https://pixlr.com/x/
Click [OPEN IMAGE] and select the file from your computer,
Click the “Crop” icon
Drag the edge controls to select part of the image or specify the new image size using the width and height in the top left corner.
When you are happy with your section click [Apply]
Click [Save] to open the download dialogue box
If cropping for a website reduce the image quality using the slider as much as possible without distorting the image. (The medium preset works well)
The image will now be saved to your default download folder