If the file type of an image is anything other than a .PNG file type, then the Instant Alpha tool in Preview will not work. To enable the Instant Alpha function, please convert to a PNG by following the steps below: 

1. Open the file in Preview , and go to File and then click on Export.
   
    
2. At this stage, there is the option to change the location and name of the file, you may wish to do this. Under Format, select PNG, and click Save. 
   
    
3. Open the New PNG file, and select the Toolbox Icon , and then select the Instant Alpha Icon .
   
    
4. Place your mouse over the part of the image you wish to isolate/remove, and Click and Drag. As you drag, surrounding areas with similar colouring will become highlighted pink. 
   
    
5. Press the Delete Key to remove the highlighted areas. Repeat the process as necessary.