Step 1
Open the image
Duplicate the layer

Step 2
Delete background layer

Step 3
Make a rectangle selection of the white background.

Step 4
Use the selection menu and Modify/Select similar.

Step 5
Delete, using the  Edit/Clear menu or by hitting the 'Delete' key on your keyboard.

Step 6
Resize using image/resize menu then, using the Pixel size area - enter 255 in the height or width with the largest number.

Step 7
Sharpen with the sharpen filter one time.

Step 8
Use the selection menu;  'Select all'

Step 9
Click in the image one time to set the selection.