Click on the image you want to crop.

Position the cursor where you would like to begin your crop and click and hold.

Drag over the area that you want to crop.

Press return on your keyboard.  The image will be cropped.

If you would like to resize the image after cropping, then just select the new size from the drop-down on the right. This affects the size the image will be exported at. It does not resize the actual image on disk.