Trying to have objects only intersect with image and not the transparency in the rectangle of the image

Question

I tried looking around for this quite a bit but i don't really know how to word it. Right now I have my code to detect wether two UIimages intersect but it checks if the square around the image intersects. The image is a triangle shape so that doesn't work well. Where should I look to have it only detect that the images have intersected if the actual images intersect and not the transparency in the square around each image. Thank you!