Applying border to image shape

In my application I am having various images of different different shapes. Like tree, cloud. (Sample image is attached). I want to add border to those shapes pro-grammatically. Like if image is of tree then need to highlight tree shape. I cannot use calayer as it will apply border to UIImageView. Can anyone guide me how to achieve this?

Simple option is to draw the image twice, first with a small scale applied to grow the image a little. Masking if the images aren't transparent (but are black&white).