how to use rsizr
Choose the "crop" tab to access this function. Cropping an image allows you to reframe the image by removing unwanted areas along the sides. To crop, click to indicate one corner of the desired crop area and drag across your image until you arrive at the opposing corner:
You can see the dimensions (in pixels) of your current crop area in the submenu. Once a crop area is defined, you can reposition it by clicking and dragging it. Click the Crop button to crop at the indicated crop area.
You can now go on to crop this cropped image, if you like. Or, if you want to start over with your uncropped image, click Reset. For precise crops, you can zoom in using the zoom in and out buttons or using your mouse's wheel:
   opening pictures

   rsizr interface

   resizing pictures
    > cropping

   saving pictures