I have solved it now!
I drew a rectangle around the image, and on the mouse move event, I retrieved the current pixel from the cursor location, and then got the Alpha channel from the colour. Then just checked for when the Alpha channel was not equal to 0.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…