If enabled, any images which are larger than the height and width specified will be resized to fit the size limit.
This is highly recommended for images intended for normal display in a web browser. Very large images (e.g. straight from a digital camera) are slow to load in the browser and take up disk space unnecessarily, quickly using up your quota. There is also no loss in quality when viewing on a typical monitor.
However, you may wish to upload the original without resizing if you are sharing an image for printing.
You can choose the default setting for this option on your Settings page.